Authorities say 33-year-old Jorge Herrera fell during the eighth race Thursday afternoon and may have been hit by his own or another horse. He suffered head injuries and died at a hospital.
Video of the race and witnesses indicate that Herrera was thrown after his horse, Morito, clipped another horse.
Herrera was from Jalisco, Mexico. He started in more than 1,000 races and won 55.
He's the first jockey to die at the Alameda County Fairgrounds since 1975.