Although previous scholarship has examined the relationship between religious involvement and a wide range of family outcomes, the relationship between religion and extramarital sexual behavior remains understudied. The authors investigate how religious affiliation, participation, and biblical beliefs explain differences in self-reported marital infidelity. This study examines data from the 1991-2004 General Social Surveys and finds that religious factors are associated with the likelihood of marital infidelity. Both church attendance and biblical beliefs are associated with lower odds of self-reported infidelity. Additionally, the authors find substantial denominational variations in the odds of marital infidelity, particularly among those who strongly affiliate with their religious group. (Contains 4 tables, 4 notes, and an appendix listing the classification of denominations.)