Neither the pilot nor the Border Patrol observer was injured.
The A-Star helicopter was two miles west of the U.S. Port of Entry (search) in Andrade, Calif., on Tuesday when a group of immigrants began throwing rocks at the aircraft.
One baseball-sized rock gashed the rotor, forcing the pilot to land nearby, said Michael Gramley, spokesman for the Border Patrol sector based in Yuma, Ariz.
Gramley said he did not know how high the helicopter was hovering when it was struck, but he said it was being repaired and is expected back in service soon.
After the incident, 17 people were apprehended for illegally crossing the border, and two of them were being investigated for smuggling. Ten immigrants evaded capture.
The investigation into the rock-throwing was continuing.