Officials say eight California prison guards and seven inmates were taken to hospitals after a fight between two inmates quickly raged out of control.
Correctional officers fired 19 bullets from semi-automatic rifles and three hard foam rounds to break up Wednesday's melee at Pelican Bay State Prison near the Oregon border.
Five of the seven injured inmates suffered gunshot wounds. Their conditions weren't immediately known.
Six of the eight injured guards were treated and released. Two are hospitalized with what officials called significant injuries but were expected to be released soon.
Officials say the melee began when two inmates refused to break up a fistfight at the high-security prison housing about 2,000 inmates.
Large groups of prisoners then ran toward the fight and attacked the guards.