As coral reefs are home to dense aggregations of a variety of species, aggressive territoriality is often a critical component of individual behavior. Identification and assessment of the risk posed by intruders is crucial to defending a territory, and fishes on coral reefs have been found to attend to body shape, body size, and coloration when responding to intruders. We examined the extent to which dusky damselfish (Stegastes adustus) discriminate among distinct categories of intruders by measuring the distance at which a fish attacks an intruder and the relative intensity and frequency of those attacks. We found that S. adustus discriminated among perceived threats, attacking conspecifics more intensely and more often than egg-predators and herbivores, and showing a trend of attacking those groups more often than invertebrate-feeders, which do not compete with damselfish for resources. Furthermore, territory holders attacked initial-phase wrasses from a farther distance than terminal-phase wrasses, suggesting that they can discriminate among classes of individuals within a species other than their own. Dusky damselfish thus exhibit the ability to make fine distinctions among intruders in a diverse ecosystem.