The Los Angeles Times reported the bus overturned shortly after 6:30 p.m. on California 38 near Yucaipa, a city about 80 miles east of Los Angeles.
Officials said a number of people were thrown from the bus when it rolled. Twenty-seven people were transported to hospitals, some in critical condition, the newspaper said.
The highway was described as a narrow mountain road leading to Big Bear but the cause of the crash and the name of the bus company weren't known.
Swim Week Miami Beach 2014 [PHOTOS]