Agonistic interactions are present throughout the animal kingdom as well as in humans. In this report, we present a model system to study neurological correlates of dominant-subordinate relationships. Zebrafish, Danio rerio, has been used as a model system for developmental biology for decades. We propose here that it is also an excellent model for studying social behavior. Adult male zebrafish were separated for 5 days and then pairs were formed and allowed to interact for 5 days. Under these conditions, aggression is prevalent and dominant-subordinate relationships are quickly established. Dominant behavior is characterized by a repeated pattern of chasing and biting, whereas subordinates engage in retreats. By day 5, the dominant-subordinate relationship was firmly established and there were differences in behavior over time. Chases, bites and retreats were all less frequent on day 5 of the social interaction than on day 1. Arginine vasotocin is the teleostean homologue of arginine vasopressin, a neuropeptide whose expression has been linked to aggression and social position in mammals. Immunohistochemistry indicated differences in vasotocin staining between dominant and subordinate individuals. Dominant individuals express vasotocin in one to three pairs of large cells in the magnocellular preoptic area whereas subordinate individuals express vasotocin in 7-11 pairs of small cells in the parvocellular preoptic area. These results suggest that the vasotocinergic system may play a role in shaping dominant-subordinate relationships and agonistic behavior in this model organism.