The U shape that has been traced out by the crude birth rate in the United States and Canada is well known. Falling birth rates reached a low point in the mid-1930’s; the rate rose to a peak in 1947 and remained high through the 1950’s. In terms of cohorts, completed family size was smallest for women born around 1910, whose childbearing was concentrated in the 1930’s. With data from the 1961 census of Canada, trends in cohort marital fertility by religion are examined. The U pattern appears for both Protestants and Jews. For Catholics, a reversal in the downward trend of family size had not appeared by 1961, although the U pattern can be discerned for some subgroups such as Catholics living in big cities and persons of Irish ancestry. In the United States, however, changes in family size for all three religious groups and both whites and nonwhites follow the U pattern. Religious differentials in family size in Canada have been decreasing, but they remain much larger than either religious or color differentials in the United States, which show no decrease. The distinctive features of Catholic fertility in Canada are most pronounced among the regionallyconcentrated French Canadians, suggesting an interplay of religious, regional, and ethnic influences.