diff --git a/pkgs/development/compilers/gcc/4.9/default.nix b/pkgs/development/compilers/gcc/4.9/default.nix
index 9069387762fd..cf67d880a2cf 100644
--- a/pkgs/development/compilers/gcc/4.9/default.nix
+++ b/pkgs/development/compilers/gcc/4.9/default.nix
@@ -61,7 +61,10 @@ let version = "4.9.3";
     # Whether building a cross-compiler for GNU/Hurd.
     crossGNU = cross != null && cross.config == "i586-pc-gnu";
 
-    enableParallelBuilding = true;
+    # Builds of gfortran have failed with strange errors that we cannot reproduce
+    # (http://hydra.nixos.org/build/23951123). Our best guess is that the build
+    # system has bugs that are exposed by compiling with multiple threads.
+    enableParallelBuilding = !langFortran;
 
     patches = [ ]
       ++ optional enableParallelBuilding ../parallel-bconfig.patch