The determinants of modern contraceptive use in traditional populations are analysed in married women aged 30–44 living in the province of Marrakech (Morocco). Women who have never used contraception have smaller family sizes than those who do: the number of live children (or live births) is the variable with maximum predictive power on contraceptive use, while child mortality is the main inhibiting factor. The probability of contraceptive use increases with female age at marriage and decreases with the woman's age, indicating a generational change in reproductive behaviour. The socioeconomic variables education, employment and residence, have no significant independent predictive character on contraceptive use, although the interaction between education and residence does. The paper evaluates the hypothesis that traditional populations in the initial phase of their demographic transition resort to modern contraception in order to stop childbearing, when they have reached a desired number of children, rather than to space births or reduce their fertility.