Two hundred twenty-two individuals of the southern plains woodrat (Neotoma micropus) were captured from 198 excavated middens at 10 discrete collecting sites from a single population in south-central Texas. Field data, mitochondrial D-loop haplotypes, and polymorphic microsatellite loci (5–7) were used to determine genetic patterns in parentage, relatedness, and mating strategy. Microsatellite loci were highly polymorphic (average observed heterozygosity  =  0.859) and were used to construct genotypes that were unique for each individual (probability of identical genotypes: 1 in 2,104,567). Results indicated a high frequency of multiple paternity (6 of 9 litters), evidence of repeat mating between the same 2 individuals, and no indication of male dominance at any collection site. Examination of these data suggested a promiscuous mating system. Within a site, average relatedness between adult females was similar to that between adult males. A higher level of cohabitation from that previously documented was recorded and finer-scale analyses revealed high levels of relatedness between most cohabiting individuals. Taken with results from other studies of mating behaviors of N. micropus, our results suggest that mating and social behavior of this species are likely influenced by population density.