Paris Hilton (search) is hot ... over something Nicole Richie (search) did.

But the hotel heiress isn't saying what's behind the feud with her co-"Simple Life" (search) star.

"It's no big secret that Nicole and I are no longer friends," Hilton said in a statement Wednesday. "Nicole knows what she did, and that's all I'm ever going to say about it."

Last week, Hilton, 24, told USA Today that Richie will be replaced on "The Simple Life" by her friend Kimberly Stewart (search), the daughter of singer Rod Stewart. Hilton and Richie have starred on the Fox reality show for its first three seasons.

Fox denied the casting change, saying in a statement: "We are currently discussing internally the creative direction of the next `The Simple Life.' We are huge fans of both Paris Hilton and Nicole Richie and look forward to solidifying our plans for the next installment."

A call to Richie's publicist was not immediately returned.

In "The Simple Life 3: Interns," Hilton and Richie took their fish-out-of-bottled-water show to the business world.

Richie, the 23-year-old daughter of R&B crooner Lionel Richie, announced her engagement to club DJ Adam Goldstein in February.