diff --git a/src/nix-build/nix-build.cc b/src/nix-build/nix-build.cc
index b4206033c..065447684 100755
--- a/src/nix-build/nix-build.cc
+++ b/src/nix-build/nix-build.cc
@@ -406,6 +406,7 @@ int main(int argc, char ** argv)
 
                 env["NIX_BUILD_TOP"] = env["TMPDIR"] = env["TEMPDIR"] = env["TMP"] = env["TEMP"] = tmp;
                 env["NIX_STORE"] = store->storeDir;
+                env["NIX_BUILD_CORES"] = settings.buildCores;
 
                 for (auto & var : drv.env)
                     env[var.first] = var.second;