Mexico's National Immigration Institute originally reported 18 dead, but later lowered that figure. Prosecutors in the southern state of Oaxaca later said there had been an overcount due to some of the bodies being dismembered, and that the real death toll was 16, AP reported.
Both sources said the dead include two women and three children, and that 29 people were injured. There was no immediate information on their condition.
Photos from the scene showed the bus rolled over onto its side on a curvy section of highway in the southern state of Oaxaca. The cause of the crash in the town of Tepelmeme, near the border with the neighboring state of Puebla, is under investigation.
The institute said a total of 55 migrants, mostly from Venezuela, were aboard the vehicle.
It was the latest in a series of migrant deaths in Mexico amid a surge in migrants traveling toward the US border. Because migration agents often raid regular buses, migrants and smugglers often seek out risky forms of transportation, like unregulated buses, trains or freight trucks.
Last week, 10 Cuban migrants died and 17 others were seriously injured after a freight truck they were riding in crashed on a highway in the neighboring state of Chiapas, near the border with Guatemala.
The National Immigration Institute said all of the dead Cuban migrants were women, and one of them was under 18.
The Institute said the driver of the vehicle had apparently been speeding and lost control of the truck, which was carrying 27 migrants at the time. The driver fled the scene.
Mexican authorities generally prohibit migrants without proper documents from buying tickets for regular buses, so those without the money to hire smugglers often hire poorly-driven, poorly-maintained buses that speed to avoid being stopped. Or they walk along the side of highways, hitching rides aboard passing trucks.
Last week, a truck flipped over on a highway in Chiapas, killing two Central American migrants and injuring another 27. And two Central American migrants died last week after trying to board a moving train in the state of Coahuila near the Texas border.