Genetic relatedness estimates and capture records were used to examine population ecology of endangered Alabama beach mice (Peromyscus polionotus ammobates). Because of their monogamous mating strategy, biparental natal care, and limited dispersal patterns, we hypothesized that beach mice form gamodemes with family groups representing the core units. Under this hypothesis, we predicted that groups of related individuals would utilize common habitat spaces and that nonrandom age or sex associations could occur. To test these predictions, livetrapping data from an 8-year study were used to examine demography and relatedness of mice involved in joint and single captures (4,683 capture events involving 1,404 individuals, of which 949 were genotyped using microsatellite data). Mice were observed in joint captures nonrandomly with respect to age and sex, implying that there are nonrandom spatial associations among beach mice. Specifically, adult females were captured more often than expected with adult males and subadults were involved in joint captures more often than expected. Subadults captured with adults were the most related cocaptures, which, along with parental assignment, provided evidence of familial clustering. Adult male–female cocaptures were among the least related pairings, suggesting that inbreeding avoidance may play a role in mate choice.