Brit against the port-a-potty!

In a Jackass segment that aired on Tuesday's Jimmy Kimmel Live, Britney Spears faced off against an icky adversary: poop.

Before performing live on the show, Spears, 29, joined Jackass star Johnny Knoxville and his raunchy pals in a pre-taped clip -- where the crew locked the pop star in a porta-potty, primed to launch into the air via bungee ropes.

Wearing goggles, Spears screams inside the portable toilet as it bounces up and down -- as what is purportedly human excrement splatters inside.

Spears' take on the toxic stunt?

"Just did my first @jackassworld stunt with Knoxville and company. Knoxville better watch his back," Tweeted the singer, whose new album, Femme Fatale, is out now.