Aggression is an innate behavior that is important for animal survival and evolution. We examined the molecular and cellular mechanisms underlying aggression in Drosophila. Reduction of the neurotransmitter octopamine, the insect equivalent of norepinephrine, decreased aggression in both males and females. Mutants lacking octopamine did not initiate fighting and did not fight other flies, although they still provoked other flies to fight themselves. Mutant males lost to the wild-type males in fighting and in competing for copulation with females. Enhanced octopaminergic signaling increased aggression in socially grouped flies, but not in socially isolated flies. We carried out genetic rescue experiments that revealed the functional importance of neuronal octopamine and identified a small subset of octopaminergic neurons in the suboesophageal ganglion as being important for aggression.