•We pharmacologically altered brain 5-HT levels using the precursor 5-HTP.•We found heightened 5-HT increased the probability of winning aggressive contests.•High-intensity behaviors and retreat threshold are influenced by 5-HT.•Other biogenic amines including octopamine remained unaltered following fights. The outcome of behavioral interactions between organisms can have significant fitness implications. Therefore, it is of great theoretical and practical importance to understand the mechanisms that modify different agonistic behaviors. Changes in central monoamines, such as serotonin (5-HT), contribute to modifying the expression of aggressive encounters in both vertebrates and invertebrates. In several invertebrate groups, neural 5-HT has been linked to heightened aggression and conflict escalation. The male stalk-eyed fly (Teleopsis dalmanni) competes with conspecifics daily over access to resources such as food and mates. Because encounters escalate in a stereotypical manner, stalk-eyed flies provide an excellent model system to study behavioral syndromes. We hypothesized that noninvasive, pharmacological augmentation of brain 5-HT by administration of the precursor, 5-hydroxytryptophan (5-HTP), would increase stereotypic behavioral escalation and the probability of winning a conflict over food. Size-matched male 5-HTP-treated and untreated flies were placed in a forced-fight paradigm and their aggressive behaviors scored. Individuals with higher brain 5-HT levels had a markedly higher probability of winning the contests, displayed greater levels of high-intensity aggressive behaviors and fewer retreats. Pretreatment with 5-HTP did not significantly alter octopamine or tyramine, suggesting that central 5-HT may modulate aggression in these organisms and play a role in determining reproductive success and resource attainment.