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American black bears (Ursus americanus (Pallas, 1780)) are characterized by female philopatry and male-biased dispersal, with predictable consequences for genetic structure of populations. We studied a recolonizing population of black bears on a desert montane island to test genetic-based predictions of bear social behavior. We assessed,genetic paternity and relatedness among bears within Big Bend National Park, Texas, from 1998 to 2001 via maternally and biparentally inherited markers and field observations. Data from seven microsatellite loci permitted us to assign paternity for 7 of 12 cubs, and multiple paternity was revealed in one litter. Levels of relatedness in the Park were comparable to those found in a nearby large population in Coahuila, Mexico. Adult female bears in the Park were more closely related to each other than males were to each other. Microsatellite data were consistent with previous analyses of mtDNA sequences that indicated bears in the Mexico-Texas metapopulation exhibit male-biased dispersal. Demographic and genetic data provided a pedigree for 23 of 31 sampled bears and depicted the matriarchal structure of this recently recolonized population. Although females in this insular population are closely related to each other, as predicted by characteristics of ursine social ecology, incoming dispersal by unrelated males results in periodic supplementation of genetic variation.