Nayarit state prosecutor's spokesman Leonel Escobedo said three children, two boys and a girl, were among the dead, though he didn't have their ages. Another 31 people were injured in the accident in the town of Tequepexpan.
The passengers were vacationers from the northern state of Chihuahua who were heading to Guayabitos beach in western Nayarit state, authorities said.
The preliminary investigation shows the bus driver, who was among the dead, was unfamiliar with the highway and appeared to be traveling too fast, Escobedo said. He also said rain overnight and a road slick may have been a factor, making it difficult for the driver to brake.