> Kernel.should_receive(:system).with(/omssa_aws2ez2_unix\.pl/).and_return(true) > is what you're looking for. erm, that's a tad embarrasing :) I copy/pasted your line without editing it! It should be: OmssaPacker.should_receive(:system).with(/omssa_aws2ez2_unix\.pl/).and_return(true) Pat