The effects of religion on the fertility patterns of Mexican Americans are examined with two different path models, the Institutional Model using formal affiliation with the Roman Catholic Church as a measure of religion, and the Religiosity Model using a measure of religiosity. Each model, tested separately for husbands and wives, examines the effects of religion on types of contraceptive methods used and on wanted family size. Although the majority of Mexican Americans are Catholics and tend to have large families, religion does not seem to have the same effect on their fertility patterns as on that of other Catholics in the United States. Among the men, neither formal affiliation nor religiosity affect the fertility patterns in any way, while among the women the effect is slight. Considering the Catholic Church’s position on contraceptive usage, it is especially noteworthy that religion does not affect the use or non-use of the more effective means of contraception, a factor contributing to the generally weak association between the measures of religion and wanted family size. The last section attempts a partial explanation of why the results turned out as they did.