Serotonin has been shown to influence aggression in many species of animals. In fruit flies, using genetic methods, stimulation of the total population of serotonin neurons (about 100), enhances aggression. Serotonin is not, however, required for the aggression seen and little is known of the neuronal pathway involved. Using additional powerful genetic methods, we have found a pair of serotonin neurons on each side of the brain that duplicate the total effect seen when stimulating the entire population. These neurons send processes to a small cluster of optic neuronal processes (called a glomerulus) that in turn sends single cholinergic (excitatory) and GABAergic (inhibitory) neurons to the next synaptic region in the aggression circuitry. Stimulation of the pairs of serotonin neurons we have found influence the choice between inhibition and excitation by a “switch-like” mechanism. Using maps of neurons and their synaptic processes described in a recent Janelia Farm publication should facilitate the further elaboration of this pathway. Completion of this circuitry should yield valuable information on how aggression is processed in the nervous system, at least of fruit flies.