Based on a series of correlations and the results of several experimental series on the territorial behavior of the threespot damselfish Eupomacentrus planifrons, a two-component system of the causation of territorial aggression is proposed. The first component, spatial commitment, determines the size of the area defended; the second, attack readiness, determines the vigor of such defense. The two differ in their responses to such factors as social isolation and repeated conspecific stimulation (and stimulation from non-related fishes). By their interaction, these two independent systems can account for not only many of the apparently contradictory results reported in the literature concerning the causation of aggressive behavior (such as the effects of social isolation), but also the stability of social systems based on territoriality. Forms of territorial advertisement, such as bird song and poster-coloration, are considered in terms of this two-component system.