The southern elephant seal, Mirounga leonina, exhibits extreme sexual dimorphism and polygyny and is thought to be an ideal subject to test maternal investment theory. Predictions concerning differential investment in offspring by sex were tested on M. leonina breeding at Marion Island over three austral summers. Large females produced more male pups, while small females produced more female pups, providing tentative support for the Trivers and Willard (1973) hypothesis. Maternal size had a greater influence on growth rate and weaning mass than on the sex of offspring. Differential reproductive costs to adult females were not evident in terms of future fecundity or survival. In keeping with other studies on this species, we could not demonstrate any differential investment in the two sexes. We suggest that before discounting maternal investment theories, further studies need to be undertaken to determine the benefits, if any, of size at weaning on long-term survival and reproductive success.