We evaluated breeding season and breeding behavior of two populations (Los Angeles and La Perforadora) of the Mexican prairie dog (Cynomys mexicanus) in northeastern Mexico during January–June 2002 and 2003. We observed 18 coteries in each colony. The breeding season began at the end of January and extended into late April for both colonies. Males were sexually active January–March, and females were in estrus in early March. However, 7 and 10 mature females in La Perforadora and Los Angeles, respectively, underwent estrus in mid-February. Gestation lasted from early March to early April, parturition occurred in early April, and weaning occurred in late April. Infanticide was not observed. Mature females and dominant males of each coterie cut fresh vegetation, possibly for construction of the nest inside the burrow. A series of sexual-mounting exercises occurred outside burrows between young of the same coterie. We describe fights that occurred frequently between adults after the young emerged from their burrows.