diff --git a/pkgs/development/tools/rust/rustup/default.nix b/pkgs/development/tools/rust/rustup/default.nix
index f683e5021415..911a956b9c44 100644
--- a/pkgs/development/tools/rust/rustup/default.nix
+++ b/pkgs/development/tools/rust/rustup/default.nix
@@ -79,6 +79,10 @@ rustPlatform.buildRustPackage (finalAttrs: {
   # Random tests fail nondeterministically on macOS.
   # TODO: Investigate this.
   doCheck = !stdenv.hostPlatform.isDarwin;
+  # Random failures when running tests in parallel.
+  preCheck = ''
+    export NIX_BUILD_CORES=1
+  '';
 
   # skip failing tests
   checkFlags = [