Mortality risks under age five are estimated using data from the 1990 Nigerian Demographic and Health Survey for children in monogamous and polygynous families. Integrating existing theories on polygyny’s relationship with infant and child mortality and some demographic concepts, the study shows that polygyny has different effects on infant and child mortality at different ages. The results indicate that polygyny does not have a significant effect on neonatal mortality (age less than one month). In contrast to the results of previous research, polygyny is significantly associated with lower child mortality during the post-neonatal period (1–11 months), but not during childhood (12–59 months). The study found socio-economic factors to be important confounders of the relationship between polygyny and mortality during the neonatal and post-neonatal periods. The protective effect of polygyny during the post-neonatal period suggests the need to further investigate circumstances that may favour post-neonatal child survival in polygynous families including availability of childcare.