Credit: AKM Images Inc

It's almost "Criminal" how good Britney Spears looks in a bikini.

The 29-year-old pop star showed off her toned tummy and lean legs in a skimpy lime green two-piece in Rio de Janeiro, Brazil, Monday. Later that evening, Spears kicked off the South American leg of her Femme Fatale tour -- her 66th show this year.

Spears was joined by her longtime love Jason Trawick, 39, and sons Sean Preston, 6, and Jayden James, 5, (with ex-husband Kevin Federline). She was also accompanied by her mother, Lynne, 56.

How does the princess of pop stay in such great shape?

"I have a trainer. I run on the treadmill for 30 minutes," the star told Ryan Seacrest earlier this year. "We do a lot of cardio and running. He kicks my butt!" See more pictures of Brit's bikini bod in Rio.

Spears also exercises with Trawick, who shed a whopping 50 pounds in two years. "We love to work out," she said. "We do stuff together like that. We take walks."