This study assesses the empirical evidence for the presence and the pattern of heterogeneity in child mortality between families in a rural area of Senegal that is apparently homogeneous with regard to the known determinants of mortality. The presence of heterogeneity is assessed by comparing the distribution of child deaths for women with a chance distribution. To weigh the relative contribution of causes of within-and between-family heterogeneity in the risk of child death to the observed pattern of clustering, detailed simulations in which empirical child-based data from the study are used, are undertaken. There are important variations in child mortality between families in this community, and the familial component in child mortality is almost entirely correlated with the size of the family. High-risk women experience repeated child deaths and repeated short birth intervals; they therefore have larger families. Heterogeneity in mortality and selective fertility, however, fail to explain the observed association between low mortality risk and small family size.