Mustaine recently complained about a recent Metallica documentary he appeared in - to discuss his brief stint in the group - insisting it made him look stupid.
But Ulrich has snapped back, claiming the singer should stop talking about a band he was in for just two years, never made a record with, and left in 1983.
Ulrich says, "I'm a little perplexed by his reaction.
"There's this weird thing that's kind of sad and pathetic that, X percentage of the press around a new Megadeth release simply becomes about Metallica.
"Remember that the guy was in our band for just around a year. He's never played on a Metallica record. It was 22 years ago. It's pretty absurd that it still can be that big a deal."