A former McDonald's employee claims mice ran rampant in the Philadelphia fast food stop where he worked, and has video of one mouse scurrying across Big Mac buns inside the McDonald's bun bag to prove it.

'I heard some rustling.'

Karrium Demaio, 29, was working in the back when he heard something moving in a bun bag of McDonald's Big Mac buns. I was going back there to get something else, and I heard some rustling, he  told Fox News.

Moving closer, he saw a large mouse inside the plastic bag, running over and between the hamburger buns in an attempt to escape. Not on top of the package, he marveled. But inside the package.

The mouse scrabbled over the top of the buns, squeezed in between their sides, and wriggled around the bag's edges.

Before he got the mouse out, Demaio shot footage of the mouse on his phone, a video that has since gone viral.

Demaio Told to Brush Off Droppings, Serve Buns

A mouse in a bun bag isn't that unusual for a fast food restaurant.

It's gross and unsanitary, sure. But as long as pests are kept under control and the food contaminated by the rodent never reaches a customer, McDonald's isn't expect to do much more.

But according to Demaio, who says he took the video to warn future customers, mice are a common occurence at the Philadelphia fast food stop.

There hasn't been a time when we couldn't go in the back and see mouse droppings on the bread, he said.

And once he'd gotten the mouse out of the bag, Demaio claims that his supervisor told him to brush off the mouse droppings and serve any un-nibbled buns to McDonald's customers.

Karrium Demaio asserts that the mouse in the bun bag video is only one of several rodent invasions at the McDonald's where he worked from October 2010 to this January.

That wasn't the first time. That was about the sixth or seventh time, said Demaio. That's what made me like, I got to get video of this.

'I am taking this matter seriously.'

Demaio was fired from the fast food restaurant this month for skipping a shift. He told Fox News he'd worked at two other McDonald's in the area, but had never seen mice there.

Demaio's supervisor refused to comment. After viewing the video, however, the McDonald's owner and operator Ken Youngblood released a statement assuring customers that he was investigating the incident.

I want my customers to know that I am taking this matter seriously, and will immediately address any issues that may exist, he said.

If necessary, we will work with the appropriate authorities to get the facts.

Video: Mouse Trapped in Big Mac Bag

Watch Mouse Struggling in a Bag of McDonald's Big Mac Buns Below: