The spread of fertility control differs from religion to religion. For instance, the decline in marital fertility among Catholics in countries including Germany and The Netherlands was delayed compared with that in other religious groups (Knodel 1974; Van Poppel 1985), while Jews are thought to have led the rest of the population in achieving lower levels of reproduction in countries like Germany and Italy (Knodel 1974; Livi Bacci 1986). As yet, much needs to be learned about the proximate and ultimate determinants of religious differentials in fertility. In this paper, these determinants are investigated in a Dutch setting, using birth-history data from the population registers of The Hague, which are available from 1850. Our questions are, first, whether religious differentials can be explained by socioeconomic characteristics; second, to what extent religious values explain the behavior of religious groups; third, which proximate determinants account for the religious differentials; and fourth, whether Jews were "forerunners" in the fertility transition. Since religious differentials and their determinants may change over time, we examine the first three questions for two different periods - before and during the transition.