Their hands and feet were also tied, said Juan Mateo Rodriguez, an Oaxaca state police deputy from the Cuenca del Papaloapan region.
Several bodies were inside an abandoned van, while others were near the vehicle.
"Farmers who had been going to work around the town of Cosolapa found the bodies and alerted us," said Rodriguez.
The town, home to 15,000 people, is in Oaxaca, one of Mexico's poorest states and the scene of violence linked to drug cartels' battles for control of lucrative drug trade routes to the United States.