diff --git a/src/pkg/io/io.go b/src/pkg/io/io.go
index bbfa6c2b2a..3e2321ce71 100644
--- a/src/pkg/io/io.go
+++ b/src/pkg/io/io.go
@@ -6,6 +6,10 @@
 // Its primary job is to wrap existing implementations of such primitives,
 // such as those in package os, into shared public interfaces that
 // abstract the functionality, plus some other related primitives.
+//
+// Because these interfaces and primitives wrap lower-level operations with
+// various implementations, unless otherwise informed clients should not
+// assume they are safe for parallel execution.
 package io
 
 import (