The aim of this study is to look at some determinants of fertility differences between monogamous and polygynous wives of ranks 1 to 3 or higher in two rural regions of Senegal. The measure of fertility is a dichotomous variable that refers to the occurrence of a birth during the 12 months prior to the census date. The analysis of cross-sectional data for the rural Tambacounda and Kolda regions from the 1988 Senegal census allowed us to test our hypotheses and to find the following results: first, the fertility of each wife decreases with the number of wives in the union; second, the wife of highest rank is more likely to have given birth in the previous year than her co-wives; as for the age of the husband, it appears to have a stronger effect for monogamists than for polygynists, for whom it is substantial only after 60; childbearing by one wife during the previous year increases the probability of a birth to a co-wife; finally, the presence of a first wife past the age of childbearing has no effect on the fertility of her co-wives.