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.
Astronomers offer more expansive view of universe
Duggar sisters unveil Christian dating rules in new book