Differences in behavioral observability, as determined by the prevalence of religious endogamy, account for differences in compliance with the religious proscription against the use of artifical means of birth control. Data are presented on fertility differences among Negro women members of Roman Catholic and Pentecostal-Holiness churches. Differences in behavioral observability for these women are determined by differences in the church membership status of their husbands. High behavioral observability systematically predicts high fertility. These findings suggest that a stronger emphasis on the explanatory power of situational or external constraints would be profitable in the study of religious organizations.