Kinship has been shown to play a crucial role in shaping the social structure of animal societies.We examined the genetic relationships of adult and sub-adult males (N = 17) and females (N = 15) from five social groups of black howler monkeys (Alouatta pigra) at Palenque National Park, Mexico, by genotyping each individual at 21 microsatellite markers. These findings were related to patterns of intragroup spatial associations and affiliative and agonistic interactions recorded over a 28-month period of behavioural observation in the field.We demonstrate that the social structure of this black howler monkey population is dominated by strong social relationships and high degrees of genetic relatedness among females. Female kin had stronger relationships because they were less aggressive to each other than female non-kin. Nevertheless, females resident in the same social group frequently spent time close to one another and affiliated with each other regardless of kinship. Relationships among males from the same social group were based on avoidance and tolerance, as males rarely interacted either affiliatively or agonistically and spent limited time close to one another. Nonetheless, kinship was a significant predictor of agonistic interactions among males, with unrelated or distantly related males engaging in agonism at higher rates than close male kin. Adult males and females rarely co-resided with adult kin from the opposite sex, and they affiliated and spatially associated at rates intermediary to those among females and those among males.