Pigs are important in the epidemiology of toxoplasmosis in North America. Seroprevalence of Toxoplasma gondii infection in 525 domestic pigs (337 backyard raised, 188 farm raised) in Oaxaca State, Mexico was determined using the modified agglutination test (MAT, cutoff 1:25). Antibodies to T. gondii were found in 58 (17.2%) of the 337 backyard pigs with MAT titers of 1:25 in 10, 1:50 in 12, 1:100 in 18, 1:200 in 6, 1:400 in 6, 1:800 in 3, and 1:1,600 in 3. Seropositive pigs were found in 39 (37.9%) of 103 homes in all 7 municipalities surveyed. Seroprevalence of T. gondii infection in backyard pigs varied with age, gender, geographic region, climate, and altitude. High seroprevalence was found in pigs ≥9-mo-old (40%), in females (40%), in pigs from the Istmo region (33.3%), and in those raised in a tropical climate (65%). Seroprevalence was higher (24.5%) in pigs raised at 100–660 m above sea level than in those at 20–60 m of altitude (14.2%). With respect to farm pigs, only 1 (0.5%, MAT titer 1:100) of 188 pigs from 5 farms was positive for anti-T. gondii antibodies. The results affirm that the management system (outdoor vs. indoor system with biosecurity) is a key factor in the epidemiology of porcine toxoplasmosis. Because there is no national system of determining the T. gondii infection status at the time of slaughter, precautions should be taken while handling pig carcasses, and all pork should be cooked thoroughly before human consumption.