Colonially breeding gray seals are polygynous. Males are larger than females, compete with each other for position among aggregated females, and contribute no parental care. Genetic analysis of pups born on the island of North Rona, Scotland, reveals large numbers of full siblings, although dominant males father disproportionately few of these. This result cannot be explained by mating patterns based solely on male dominance and the spatio-temporal organization of the breeding colony. Instead, many full siblings must result from choices favoring previous parental combinations. Thus, polygyny and partner fidelity appear to operate simultaneously in this breeding colony.