Derrick Holt posted a picture on Facebook and Twitter, of what he says is a half-eaten worm next to his Sausage McMuffin.
He said at first, he thought it was just a piece of fat, but when he pulled it out, he discovered it wasn't.
Derrick said he went straight to the manager - and that's when he says things got even more strange:
"I was like, I think it's a worm, and he goes, oh, no no, looks at it, picks it up, he's like that's just a bone. He says that happens all the time in our patties, in our hamburgers, in our chicken nuggets especially."
Derrick said the manager took the sandwich and gave him a refund.
McDonald's did respond to Derrick's complaint, saying they are looking into the matter. They also gave him coupons for free meals.