diff --git a/JustSaying.UnitTests/CreateMe/WhenCreatingABus.cs b/JustSaying.UnitTests/CreateMe/WhenCreatingABus.cs
index e70f4ce62..f59d9b902 100644
--- a/JustSaying.UnitTests/CreateMe/WhenCreatingABus.cs
+++ b/JustSaying.UnitTests/CreateMe/WhenCreatingABus.cs
@@ -4,6 +4,7 @@
 
 namespace JustSaying.UnitTests.CreateMe
 {
+    [CollectionDefinition(nameof(WhenCreatingABus), DisableParallelization = true)]
     public class WhenCreatingABus
     {
         private readonly Action<IPublishConfiguration> _config;
diff --git a/JustSaying.UnitTests/JustSayingFluentlyTestBase.cs b/JustSaying.UnitTests/JustSayingFluentlyTestBase.cs
index 755f607ab..28c4908e1 100644
--- a/JustSaying.UnitTests/JustSayingFluentlyTestBase.cs
+++ b/JustSaying.UnitTests/JustSayingFluentlyTestBase.cs
@@ -8,6 +8,7 @@
 
 namespace JustSaying.UnitTests
 {
+    [CollectionDefinition(nameof(JustSayingFluentlyTestBase), DisableParallelization = true)]
     public abstract class JustSayingFluentlyTestBase : XAsyncBehaviourTest<JustSaying.JustSayingFluently>
     {
         protected IPublishConfiguration Configuration;