Catholic and non-Catholic fertility during the post-World War II period are compared in this paper. Evidence accumulated across five sample surveys of fertility in the United States, which were conducted at five-year intervals from 1955 through 1975, forms the basis for the analysis; both cohort and period measures are employed. Starting from a situation where Catholic fertility was very little higher than that of non-Catholics, it is shown that the differential increased markedly during the baby boom and then declined to a point where the two trends nearly come together in the mid-1970s. Interpretation of the recent convergence in the light of various theories that have been put forward to explain the differential suggests that it will be an enduring phenomenon.