This article presents to the results of the National Seroepidemiology Survey with regard to the prevalence of toxoplasmosis in Mexico. Some theoretical aspects of the disease are discussed, and the prevalences found in earlier reports are presented. The study measured 29,279 blood samples from persons in all 32 states of Mexico, for both sexes, all ages, and socio-economic groups. The samples were processed by indirect immunofluorescence for antibody titers of 1:16 and 1:128. Positivity levels were stratified by age, sex, socioeconomic level, place of residence, education level, number of persons in the family, state, and with or without rights to care in the social security medical institutions. The results pointed to greater prevalences in the costal areas, in lower socioeconomic levels, and a high incidence in women of reproductive age. Also included are a discussion of socio-economic aspects and the potential role of the domestic cat.