This chapter explores the impact of polygynous marriage on a variety of fitness indices in the Datoga of Tanzania and attempts to delineate some of the dynamics of sexual conflict in their marriage system. The Datoga are relatively poor among East African pastoralists, child mortality rates are high, and data on morbidity, child growth, and adult body size strongly suggest high rates of infection and undernutrition. Accordingly, for yearly hazards of fertility and child mortality, an annual measure of marital status was used. Analyses of covariance were performed to test for associations between children's growth status, marital status of the mother, and indicators of household wealth while controlling for the age and sex of children. For childbearing women the effects of household wealth and modal number of co-wives on the rate of production of surviving offspring were examined. Datoga co-wives compete for access to the food resources produced in their households.