For polygynous mammals with no paternal care, the number of offspring sired is often the sole measure of male reproductive success. The potential for polygyny is highest when resources or other environmental factors such as restricted breeding sites force females to aggregate. In these circumstances, males compete intensely for females and mating success may vary greatly among males, further intensifying selection for those traits that confer an advantage in reproduction. Hence, determinants of male success in competition for females are likely to be under strong sexual selection. Paternity analysis was used in conjunction with measures of age, site fidelity, and behavior during the breeding season to assess variance in male breeding success in Weddell seals (Leptonychotes weddellii) breeding at Turtle Rock, McMurdo Sound (77.727S, 166.85E) between 1997 and 2000. Paternity could be assigned to 177 pups at relaxed or 80% confidence level or 111 pups at strict or 95% confidence levels. Weddell seals at Turtle Rock show a modest degree of polygyny with the greatest number of pups sired by any individual male in a single season equalling 5 or ∼10% of the pups born. Over four consecutive years, most (89.2%) males sired at least one pup. In a generalized linear model (GLM), age and the age first seen at the study site as an adult were unrelated to mating success, but adult experience, either site-specific or elsewhere in McMurdo Sound, over the reproductive life span of males explained nearly 40% of variance in total mating success with 80% confidence and 24% of variance at 95% confidence. While learning where females are likely to be may enhance male reproductive success, aquatic mating reduces the ability of males to monopolize females, and thereby increases equity in mating success.