Serotonin is an important neurotransmitter in the regulation of social interactions in many animals. Correlative studies in numerous vertebrate species, including fishes, indicate that aggressive males have lower relative serotonergic activity than less aggressive males. We used fluoxetine, a selective serotonin reuptake inhibitor, to experimentally enhance serotonergic neurotransmission in a territorial coral reef fish and test the role of this neurotransmitter in mediating aggressive behavior and dominance interactions. The bluehead wrasse, Thalassoma bifasciatum, has a complex social system in which large males aggressively defend spawning territories from intruders. In separate experiments, we tested the effects of chronic and acute fluoxetine treatments on aggressive behavior using a resident-intruder design. In a laboratory experiment, males treated daily with intraperitoneal fluoxetine injections for 2 weeks (6 microg/g bw) displayed fewer intruder chases than saline-treated controls. Chronically fluoxetine-treated males also showed lower levels of activity than saline controls prior to intruder trials. However, activity was not correlated with chases on an individual level, indicating the lower aggression displayed by fluoxetine-treated males was not due solely to general reductions in behavioral display. A field study exposed males to a confined territorial intruder following single intraperitoneal injections of fluoxetine (10 microg/g bw) or saline given to the same individual on different days. The frequency of aggressive chases following acute fluoxetine treatment was significantly lower than that following saline injections. This study experimentally supports the link between serotonin and aggressive behavior in fishes in both a controlled laboratory testing environment and the animal's natural habitat.