diff --git a/CHANGES.md b/CHANGES.md
index 0aabfce3..393d3565 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -24,6 +24,15 @@ installation method we recommend in the [Nextstrain installation
 documentation](https://docs.nextstrain.org/page/install.html).  In that case, a
 supported Python version is always bundled with `nextstrain`.
 
+## Improvements
+
+* The Conda runtime now uses Micromamba 1.5.8 (upgraded from 1.1.0) to manage
+  the runtime environment. The newer version supports Zstandard-compressed
+  index files which speeds up `nextstrain setup` and `nextstrain update` for
+  the Conda runtime.
+  ([#367](https://github.com/nextstrain/cli/pull/367))
+
+
 # 8.3.0 (30 April 2024)
 
 ## Improvements
diff --git a/nextstrain/cli/runner/conda.py b/nextstrain/cli/runner/conda.py
index 48cbe5e5..a850dfd0 100644
--- a/nextstrain/cli/runner/conda.py
+++ b/nextstrain/cli/runner/conda.py
@@ -67,7 +67,7 @@
     <https://anaconda.org/conda-forge/micromamba/>`__, or the special string
     ``latest``.
 
-    Defaults to ``1.1.0``.
+    Defaults to ``1.5.8``.
 """
 
 import json
@@ -100,7 +100,7 @@
 
 # If you update the version pin below, please update the docstring above too.
 MICROMAMBA_VERSION = os.environ.get("NEXTSTRAIN_CONDA_MICROMAMBA_VERSION") \
-                  or "1.1.0"
+                  or "1.5.8"
 
 NEXTSTRAIN_CHANNEL = os.environ.get("NEXTSTRAIN_CONDA_CHANNEL") \
                   or "nextstrain"