Males of most species are more aggressive than females, but the neural mechanisms underlying this dimorphism are not clear. Here, we identify a neuron and a gene that control the higher level of aggression characteristic of Drosophila melanogaster males. Males, but not females, contain a small cluster of FruM+ neurons that express the neuropeptide tachykinin (Tk). Activation and silencing of these neurons increased and decreased, respectively, intermale aggression without affecting male-female courtship behavior. Mutations in both Tk and a candidate receptor, Takr86C, suppressed the effect of neuronal activation, whereas overexpression of Tk potentiated it. Tk neuron activation overcame reduced aggressiveness caused by eliminating a variety of sensory or contextual cues, suggesting that it promotes aggressive arousal or motivation. Tachykinin/Substance P has been implicated in aggression in mammals, including humans. Thus, the higher aggressiveness of Drosophila males reflects the sexually dimorphic expression of a neuropeptide that controls agonistic behaviors across phylogeny. Display omitted •A single class of neurons is identified that promotes aggression in male flies•These neurons are FruM+ and sexually dimorphic but do not control courtship•These neurons promote aggressive arousal via release of a neuropeptide, DTK•Tk is a neuropeptide gene that controls aggression in both flies and mammals A small group of Drosophila neurons that are detectable in male, but not female, brains promote high levels of aggression via the release of a neuropeptide, Tachykinin. Homologs of this neuropeptide are implicated in mammalian aggression.