This picture of Chi Omega's sorority sisters from their Mexican-themed Halloween party has caused an uproar. (Chi Omega photo)

A sorority at Penn State is facing a backlash after a picture surfaced of its members decked out in Mexican sombreros and fake moustaches and holding signs that read "Will mow lawn for weed + beer" and "I don't cut grass. I smoke it."

The picture, from Chi Omega's Mexican-themed Halloween party, was posted to Facebook and discovered by an independent Penn news site, Onward State.

"Our chapter of Chi Omega sincerely apologizes for portraying inappropriate and untrue stereotypes," sorority spokeswoman Jessica Riccardi told student newspaper Collegian Online.

The sorority's national leadership has promised disciplinary action.

"Because the right of free expression is guaranteed under the constitution — for both behaviours we disagree with or for those that we support — we are approaching this as a teachable moment to engage this group of young women in discussions about their actions and to further educate them about diversity issues," Penn State spokeswoman Lisa Powers told CNN.