diff --git a/pkgs/build-support/fetchurl/builder.sh b/pkgs/build-support/fetchurl/builder.sh
index 511839655be0..53af63092f40 100644
--- a/pkgs/build-support/fetchurl/builder.sh
+++ b/pkgs/build-support/fetchurl/builder.sh
@@ -82,8 +82,14 @@ for url in $urls; do
             # Assume that SourceForge/GNU/kernel mirrors have better
             # bandwidth than nix.cs.uu.nl.
             preferHashedMirrors=
-            
-            for url3 in ${!varName}; do
+
+            mirrors=${!varName}
+
+            # Allow command-line override by setting NIX_MIRRORS_$site.
+            varName="NIX_MIRRORS_$site"
+            if test -n "${!varName}"; then mirrors="${!varName}"; fi
+
+            for url3 in $mirrors; do
                 urls2="$urls2 $url3$fileName";
             done
         fi
diff --git a/pkgs/build-support/fetchurl/default.nix b/pkgs/build-support/fetchurl/default.nix
index e752b9313e93..f502b8890472 100644
--- a/pkgs/build-support/fetchurl/default.nix
+++ b/pkgs/build-support/fetchurl/default.nix
@@ -28,7 +28,20 @@ assert url != "" -> urls == [];
 assert (outputHash != "" && outputHashAlgo != "")
     || md5 != "" || sha1 != "" || sha256 != "";
 
-let urls_ = if urls != [] then urls else [url]; in
+let
+
+  urls_ = if urls != [] then urls else [url];
+
+  mirrors = import ./mirrors.nix;
+
+  # Names of the master sites that are mirrored (i.e., "sourceforge",
+  # "gnu", etc.).
+  sites =
+    if builtins ? attrNames
+    then builtins.attrNames mirrors
+    else [] /* backwards compatibility */;
+  
+in
 
 stdenv.mkDerivation ({
   name =
@@ -62,10 +75,10 @@ stdenv.mkDerivation ({
     # This variable allows the user to override hashedMirrors from the
     # command-line.
     "NIX_HASHED_MIRRORS"
-  ];
+  ] ++ (map (site: "NIX_MIRRORS_${site}") sites);
 }
 
 # Pass the mirror locations to the builder.
-// (import ./mirrors.nix)
+// mirrors
 
 )