Bodies of 72 people, including 14 women, were found by Mexican marines at a ranch in Tamaulipas state, the Navy Secretariat said.
The bodies were found after an encounter between marines and gunmen in San Fernando town that left a marine and three insurgents dead, the secretariat said.
The marines entered the ranch and discovered the partially buried bodies.
Investigators suspect that the gunmen who were involved in the shooting were behind the killings at the ranch, a spokesman said.
A minor was arrested after the incident and the marines seized 21 rifles, 101 ammunition clips, four bullet-proof vests, camouflage uniforms and four vehicles.