diff --git a/pkgs/development/libraries/zlib/default.nix b/pkgs/development/libraries/zlib/default.nix
index 0938e2a17a79..60aa8d0dbeea 100644
--- a/pkgs/development/libraries/zlib/default.nix
+++ b/pkgs/development/libraries/zlib/default.nix
@@ -26,6 +26,15 @@ stdenv.mkDerivation rec {
   # to the bootstrap-tools libgcc (as uses to happen on arm/mips)
   NIX_CFLAGS_COMPILE = stdenv.lib.optionalString (!stdenv.isDarwin) "-static-libgcc";
 
+  postInstall = stdenv.lib.optionalString stdenv.isDarwin ''
+    # jww (2015-01-06): Sometimes this library install as a .so, even on
+    # Darwin; others time it installs as a .dylib.  I haven't yet figured out
+    # what causes this difference.
+    for file in $out/lib/*.so* $out/lib/*.dylib* ; do
+      install_name_tool -id "$file" $file
+    done
+  '';
+
   crossAttrs = {
     dontStrip = static;
   } // stdenv.lib.optionalAttrs (stdenv.cross.libc == "msvcrt") {