diff --git a/src/cljam/io/bam_index/reader.clj b/src/cljam/io/bam_index/reader.clj
index a13046ce..d273e349 100644
--- a/src/cljam/io/bam_index/reader.clj
+++ b/src/cljam/io/bam_index/reader.clj
@@ -2,11 +2,11 @@
   (:require [clojure.java.io :as cio]
             [cljam.io.util.lsb :as lsb]
             [cljam.io.bam-index.common :refer [bai-magic]]
-            [cljam.io.bam-index.chunk :as chunk]
+            [cljam.io.util.chunk :as chunk]
             [cljam.util :as util])
   (:import java.util.Arrays
            [java.io FileInputStream Closeable IOException]
-           [cljam.io.bam_index.chunk Chunk]))
+           [cljam.io.util.chunk Chunk]))
 
 (deftype BAIReader [url reader]
   Closeable
@@ -49,10 +49,10 @@
 (defn- read-chunks!
   [rdr]
   (let [n (lsb/read-int rdr)]
-   (loop [i 0, chunks []]
-     (if (< i n)
-       (recur (inc i) (conj chunks (Chunk. (lsb/read-long rdr) (lsb/read-long rdr))))
-       chunks))))
+    (loop [i 0, chunks []]
+      (if (< i n)
+        (recur (inc i) (conj chunks (Chunk. (lsb/read-long rdr) (lsb/read-long rdr))))
+        chunks))))
 
 (defn- read-bin-index**!
   [rdr]
diff --git a/src/cljam/io/bam_index/writer.clj b/src/cljam/io/bam_index/writer.clj
index fe8bcff7..250d76ed 100644
--- a/src/cljam/io/bam_index/writer.clj
+++ b/src/cljam/io/bam_index/writer.clj
@@ -5,12 +5,12 @@
             [cljam.io.util.bgzf :as bgzf]
             [cljam.io.util.lsb :as lsb]
             [cljam.io.bam-index.common :refer :all]
-            [cljam.io.bam-index.chunk :as chunk]
+            [cljam.io.util.chunk :as chunk]
             [cljam.io.bam.decoder :as bam-decoder])
   (:import [java.io DataOutputStream Closeable]
            [java.nio ByteBuffer ByteOrder]
            [cljam.io.bam.decoder BAMPointerBlock]
-           [cljam.io.bam_index.chunk Chunk]))
+           [cljam.io.util.chunk Chunk]))
 
 (declare make-index-from-blocks)
 
diff --git a/src/cljam/io/tabix.clj b/src/cljam/io/tabix.clj
index 9ef30172..dfd09bc9 100644
--- a/src/cljam/io/tabix.clj
+++ b/src/cljam/io/tabix.clj
@@ -3,57 +3,49 @@
   Reader of a TABIX format file."
   (:require [cljam.io.util.bgzf :as bgzf]
             [cljam.io.util.lsb :as lsb]
-            [cljam.io.util.bin :as util-bin])
+            [cljam.io.util.bin :as util-bin]
+            [clojure.string :as cstr])
   (:import java.util.Arrays
            [java.io DataInputStream IOException]
-           [cljam.io.bam_index.chunk Chunk]))
+           [cljam.io.util.chunk Chunk]))
 
 (deftype Tabix [n-ref preset sc bc ec meta skip seq bidx lidx]
   util-bin/IBinaryIndex
   (bidx-ref [this]
     (.bidx this))
   (lidx-ref [this]
-    (.lidx this)))
+    (.lidx this))
+  (get-ref-index [this chr]
+    (.indexOf
+     ^clojure.lang.PersistentVector
+     (.seq  this)
+     chr)))
 
 (def tabix-magic "TBI\1")
 
 (defn- read-chunks!
   [rdr]
-  (let [n (lsb/read-int rdr)]
-    (loop [i 0, chunks []]
-      (if (< i n)
-        (recur (inc i) (conj chunks (Chunk. (lsb/read-long rdr) (lsb/read-long rdr))))
-        chunks))))
+  (->> #(Chunk. (lsb/read-long rdr) (lsb/read-long rdr))
+       (repeatedly (lsb/read-int rdr))
+       vec))
 
 (defn- read-seq
-  [buf len]
-  (loop [i 0, j 0, seq* []]
-    (if (< i len)
-      (if (zero? (nth buf i))
-        (let [ba-len (- i j)
-              ba (byte-array ba-len)]
-          (System/arraycopy buf j ba 0 ba-len)
-          (recur (inc i) (inc i) (conj seq* (String. ba))))
-        (recur (inc i) j seq*))
-      seq*)))
+  [^bytes buf]
+  (cstr/split (String. buf) #"\00"))
 
-(defn- read-bin-index**!
+(defn- read-bin-index
   [rdr]
-  (let [n (lsb/read-int rdr)]
-    (loop [i 0, bidx []]
-      (if (< i n)
-        (let [bin (lsb/read-int rdr)
-              chunks (read-chunks! rdr)]
-          (recur (inc i) (conj bidx {:bin bin, :chunks chunks})))
-        bidx))))
+  (->> #(hash-map
+         :bin (lsb/read-int rdr)
+         :chunks (read-chunks! rdr))
+       (repeatedly (lsb/read-int rdr))
+       vec))
 
-(defn- read-linear-index**!
+(defn- read-linear-index
   [rdr]
-  (let [n (lsb/read-int rdr)]
-    (loop [i 0, lidx []]
-      (if (< i n)
-        (recur (inc i) (conj lidx (lsb/read-long rdr)))
-        lidx))))
+  (->> #(lsb/read-long rdr)
+       (repeatedly (lsb/read-int rdr))
+       vec))
 
 (defn- read-index*
   [^DataInputStream rdr]
@@ -68,14 +60,14 @@
         skip   (lsb/read-int rdr)
         len    (lsb/read-int rdr)
         buf    (lsb/read-bytes rdr len)
-        seq    (read-seq buf len)
+        seq    (read-seq buf)
         refs (range n-ref)
-        all-idx (map (fn [_] [(read-bin-index**! rdr) (read-linear-index**! rdr)]) refs)
+        all-idx (map (fn [_] [(read-bin-index rdr) (read-linear-index rdr)]) refs)
         bidx-seq (map first all-idx)
         bidx (zipmap
               refs
               (map (fn [bins]
-                     (zipmap (map :bin bins) (map :chunks bins)))
+                     (into {} (map (juxt :bin :chunks)) bins))
                    bidx-seq))
         lidx (zipmap refs (map second all-idx))]
     (->Tabix n-ref preset sc bc ec  meta
diff --git a/src/cljam/io/util/bin.clj b/src/cljam/io/util/bin.clj
index b74d6771..ab63ff8f 100644
--- a/src/cljam/io/util/bin.clj
+++ b/src/cljam/io/util/bin.clj
@@ -1,11 +1,6 @@
 (ns cljam.io.util.bin
-  (:refer-clojure :exclude [compare])
-  (:require [cljam.io.bam-index [chunk :as bam-index-chunk]
-             [writer :as bam-index-writer]])
-  (:import [java.io File]
-           [java.net MalformedURLException URI URL]
-           [cljam.io.bam_index.chunk Chunk]
-           [bgzf4j BGZFInputStream BGZFOutputStream]))
+  (:require [cljam.io.util.chunk :as util-chunk]
+            [cljam.io.bam-index.writer :as bam-index-writer]))
 
 (defn- reg->bins*
   "Returns candidate bins for the specified region as a vector."
@@ -31,15 +26,16 @@
 
 (defprotocol IBinaryIndex
   (bidx-ref [this])
-  (lidx-ref [this]))
+  (lidx-ref [this])
+  (get-ref-index [this chr]))
 
 (defn get-spans
-  [^cljam.io.util.bin.IBinaryIndex index-data ^long ref-idx ^long beg ^long end]
+  [index-data ^long ref-idx ^long beg ^long end]
   (let [bins (reg->bins beg end)
         bidx (get (bidx-ref index-data) ref-idx)
         lidx (get (lidx-ref index-data) ref-idx)
-        chunks (into [] (comp (map bidx) cat) bins)
+        chunks (into [] (mapcat bidx) bins)
         lin-beg (bam-index-writer/pos->lidx-offset beg)
         min-offset (get lidx lin-beg 0)]
-    (->> (bam-index-chunk/optimize-chunks chunks min-offset)
+    (->> (util-chunk/optimize-chunks chunks min-offset)
          (map vals))))
diff --git a/src/cljam/io/bam_index/chunk.clj b/src/cljam/io/util/chunk.clj
similarity index 82%
rename from src/cljam/io/bam_index/chunk.clj
rename to src/cljam/io/util/chunk.clj
index 65bbb5b4..7c523037 100644
--- a/src/cljam/io/bam_index/chunk.clj
+++ b/src/cljam/io/util/chunk.clj
@@ -1,4 +1,4 @@
-(ns cljam.io.bam-index.chunk
+(ns cljam.io.util.chunk
   (:refer-clojure :exclude [compare])
   (:require [cljam.io.util.bgzf :as bgzf]))
 
@@ -52,10 +52,10 @@
            ret (transient [])]
       (if f
         (cond
-         (<= (.end f) min-offset) (recur r last-chunk ret)
-         (nil? last-chunk) (recur r f (conj! ret f))
-         (and (not (overlap? last-chunk f))
-              (not (adjacent? last-chunk f))) (recur r f (conj! ret f))
-         (> (.end f) (.end last-chunk)) (let [l (assoc last-chunk :end (.end f))] (recur r l (conj! (pop! ret) l)))
-         :else (recur r last-chunk ret))
+          (<= (.end f) min-offset) (recur r last-chunk ret)
+          (nil? last-chunk) (recur r f (conj! ret f))
+          (and (not (overlap? last-chunk f))
+               (not (adjacent? last-chunk f))) (recur r f (conj! ret f))
+          (> (.end f) (.end last-chunk)) (let [l (assoc last-chunk :end (.end f))] (recur r l (conj! (pop! ret) l)))
+          :else (recur r last-chunk ret))
         (persistent! ret)))))
diff --git a/src/cljam/io/vcf.clj b/src/cljam/io/vcf.clj
index 1c09abb2..0ed95070 100644
--- a/src/cljam/io/vcf.clj
+++ b/src/cljam/io/vcf.clj
@@ -10,7 +10,8 @@
             [cljam.io.vcf.writer :as vcf-writer]
             [cljam.io.bcf.reader :as bcf-reader]
             [cljam.io.bcf.writer :as bcf-writer]
-            [cljam.io.util.bgzf :as bgzf])
+            [cljam.io.util.bgzf :as bgzf]
+            [cljam.io.tabix :as tabix])
   (:import java.io.Closeable
            cljam.io.vcf.reader.VCFReader
            cljam.io.vcf.writer.VCFWriter
@@ -30,9 +31,11 @@
         header (with-open [r (cio/reader (util/compressor-input-stream f))]
                  (vcf-reader/load-header r))]
     (VCFReader. (util/as-url f) meta-info header
+
                 (if (bgzf/bgzip? f)
                   (bgzf/bgzf-input-stream f)
-                  (cio/reader (util/compressor-input-stream f))))))
+                  (cio/reader (util/compressor-input-stream f)))
+                (delay (tabix/read-index (str f ".tbi"))))))
 
 (defn ^BCFReader bcf-reader
   "Returns an open cljam.io.bcf.reader.BCFReader of f. Should be used inside
diff --git a/src/cljam/io/vcf/reader.clj b/src/cljam/io/vcf/reader.clj
index 75c19692..b3afbcb6 100644
--- a/src/cljam/io/vcf/reader.clj
+++ b/src/cljam/io/vcf/reader.clj
@@ -7,8 +7,8 @@
             [camel-snake-kebab.core :refer [->kebab-case-keyword]]
             [proton.core :refer [as-long]]
             [cljam.io.util.bin :as util-bin]
-            [cljam.io.vcf.util :as vcf-util]
-            [cljam.io.tabix :as tabix])
+            [cljam.io.tabix :as tabix]
+            [cljam.io.vcf.util :as vcf-util])
   (:import [java.io Closeable]
            [cljam.io.tabix Tabix]
            [clojure.lang LazilyPersistentVector]
@@ -19,7 +19,7 @@
 
 (declare read-variants read-variants-randomly)
 
-(deftype VCFReader [url meta-info header reader]
+(deftype VCFReader [url meta-info header reader index-delay]
   Closeable
   (close [this]
     (.close ^Closeable (.reader this)))
@@ -183,18 +183,17 @@
                     :vcf identity)]
      (map parse-fn (read-data-lines (.reader rdr) (.header rdr) kws)))))
 
+(defn- make-lazy-variants [f s]
+  (if (not-empty s)
+    (concat
+     (f (first s))
+     (make-lazy-variants f (rest s)))))
+
 (defn read-variants-randomly
   [^VCFReader rdr {:keys [chr start end depth] :or {depth :deep start 1 end 4294967296}}]
   (let [kws (mapv keyword (drop 8 (.header rdr)))
-        tabix-path (str (.getPath ^java.net.URL (.url rdr)) ".tbi")
-        tabix-data (tabix/read-index tabix-path)
-        ref-idx (.indexOf
-                 ^clojure.lang.PersistentVector (.seq ^Tabix tabix-data)
-                 (if (and (string? chr)
-                          (> (count chr) 3)
-                          (= (subs chr 0 3) "chr"))
-                   (subs chr 3)
-                   (throw (ex-info "Invalid chr." {:chr chr}))))
+        tabix-data @(.index-delay rdr)
+        ref-idx (util-bin/get-ref-index tabix-data chr)
         spans
         (if (= ref-idx -1)
           '()
@@ -203,25 +202,21 @@
         parse-fn (case depth
                    :deep (vcf-util/variant-parser (.meta-info rdr) (.header rdr))
                    :vcf identity)]
-    (reduce
-     (fn [res span]
-       (.seek input-stream (first span))
-       (loop [res res]
-
-         (if (< (.getFilePointer input-stream) (second span))
-           (let [variant
-                 (parse-fn
-                  (parse-data-line
-                   (.readLine input-stream)
-                   kws))
-                 variant-pos (:pos variant)]
-             (if (or (< variant-pos (first span))
-                     (>= variant-pos (second span)))
-               res
-               (recur
-                (cons
-                 variant
-                 res))))
-           res)))
-     '()
+
+    (make-lazy-variants
+     (fn [[chunk-beg ^long chunk-end]]
+       (.seek input-stream chunk-beg)
+
+       (->> #(when (< (.getFilePointer input-stream) chunk-end)
+               (-> input-stream
+                   .readLine
+                   (parse-data-line kws)
+                   parse-fn))
+            repeatedly
+            (take-while identity)
+            (filter
+             (fn [{chr' :chr :keys [pos ref info]}]
+               (and (= chr' chr)
+                    (<= pos end)
+                    (<= start (get info :END (dec (+ pos (count ref))))))))))
      spans)))
diff --git a/test-resources/vcf/test-v4_3-complex.vcf.gz b/test-resources/vcf/test-v4_3-complex.vcf.gz
new file mode 100644
index 00000000..9c3520d1
Binary files /dev/null and b/test-resources/vcf/test-v4_3-complex.vcf.gz differ
diff --git a/test-resources/vcf/test-v4_3-complex.vcf.gz.tbi b/test-resources/vcf/test-v4_3-complex.vcf.gz.tbi
new file mode 100644
index 00000000..71632967
Binary files /dev/null and b/test-resources/vcf/test-v4_3-complex.vcf.gz.tbi differ
diff --git a/test/cljam/io/tabix_test.clj b/test/cljam/io/tabix_test.clj
index 214fc20c..6805d81a 100644
--- a/test/cljam/io/tabix_test.clj
+++ b/test/cljam/io/tabix_test.clj
@@ -5,13 +5,13 @@
             [cljam.io.tabix :as tbi])
   (:import
    [cljam.io.tabix Tabix]
-   [cljam.io.bam_index.chunk Chunk]))
+   [cljam.io.util.chunk Chunk]))
 
 (deftest about-read-index-with-error
   (is (thrown? java.io.IOException (tbi/read-index small-bam-file))))
 
 (deftest about-read-index-returns-tabix-object
-  (is (instance? cljam.io.tabix.Tabix (tbi/read-index test-tabix-file))))
+  (is (instance? Tabix (tbi/read-index test-tabix-file))))
 
 (deftest about-read-index-check-the-returning-object
   (let [tabix-data ^Tabix (tbi/read-index test-tabix-file)]
@@ -23,18 +23,8 @@
     (is (number? (.meta tabix-data)))
     (is (number? (.skip tabix-data)))
     (is (vector? (.seq tabix-data)))
-    (is (instance?
-         Chunk
-         (get
-          ^clojure.lang.IPersistentVector
-          (get
-           ^clojure.lang.IPersistentMap
-           (get
-            ^clojure.lang.IPersistentMap
-            (.bidx tabix-data) 0) 4687) 0)))
-    (is (vector?
-         ^clojure.lang.IPersistentVector
-         (get (.lidx tabix-data) 0)))))
+    (is (instance?  Chunk (get (get (get (.bidx tabix-data) 0) 4687) 0)))
+    (is (vector?  (get (.lidx tabix-data) 0)))))
 
 (deftest-remote large-file
   (with-before-after {:before (prepare-cavia!)}
@@ -42,8 +32,7 @@
 
 (deftest source-type-test
   (with-open [server (http-server)]
-    (are [x] ((partial instance? cljam.io.tabix.Tabix)
-              (tbi/read-index x))
+    (are [x] (instance? Tabix (tbi/read-index x))
       test-tabix-file
       (cio/file test-tabix-file)
       (cio/as-url (cio/file test-tabix-file))
diff --git a/test/cljam/io/vcf_test.clj b/test/cljam/io/vcf_test.clj
index 976b2931..ecbf636f 100644
--- a/test/cljam/io/vcf_test.clj
+++ b/test/cljam/io/vcf_test.clj
@@ -127,20 +127,42 @@
          (vcf/read-variants-randomly
           (vcf/vcf-reader
            test-large-vcf-file)
-          {:chr "chr1"
+          {:chr "1"
            :start 20
            :end 1000000})))
     (is (not-throw?
          (vcf/read-variants-randomly
           (vcf/vcf-reader
            test-large-vcf-file)
-          {:chr "chr1"
+          {:chr "1"
            :start 2000})))
     (is (not-throw?
          (vcf/read-variants-randomly
           (vcf/vcf-reader
            test-large-vcf-file)
-          {:chr "chr1"})))))
+          {:chr "1"})))))
+
+(deftest read-randomly-variants-complex-test
+  (doseq [index [{:chr "1"}
+                 {:chr "2" :start 30000}
+                 {:chr "2" :start 40000}
+                 {:chr "2" :end 40000}
+                 {:chr "2" :start 30000 :end 40000}]]
+    (with-open [vcf-file (vcf/vcf-reader test-vcf-complex-file)
+                vcf-gz-file (vcf/vcf-reader test-vcf-complex-gz-file)]
+      (is
+       (=
+        (vcf/read-variants-randomly
+         vcf-gz-file
+         index)
+        (let [{:keys [chr start end] :or {start 1 end 4294967296}} index]
+          (filter
+           (fn [variant]
+             (and (= chr (:chr variant))
+                  (<= start (:pos variant))
+                  (> end (:pos variant))))
+           (vcf/read-variants
+            vcf-file))))))))
 
 (deftest writer-test
   (testing "vcf"
diff --git a/test/cljam/test_common.clj b/test/cljam/test_common.clj
index b686487c..88bee24d 100644
--- a/test/cljam/test_common.clj
+++ b/test/cljam/test_common.clj
@@ -197,6 +197,8 @@
 (def test-vcf-v4_3-file "test-resources/vcf/test-v4_3.vcf")
 (def test-vcf-no-samples-file "test-resources/vcf/test-no-samples.vcf")
 (def test-vcf-complex-file "test-resources/vcf/test-v4_3-complex.vcf")
+(def test-vcf-complex-gz-file "test-resources/vcf/test-v4_3-complex.vcf.gz")
+(def test-vcf-complex-tbi-file "test-resources/vcf/test-v4_3-complex.vcf.gz.tbi")
 
 (def test-large-vcf-file (cavia/resource mycavia "large.vcf.gz"))
 (def test-large-vcf-tbi-file (cavia/resource mycavia "large.vcf.gz.tbi"))