SUMMARY Individual recognition in the lobster Homarus americanus(Milne-Edwards), is based on detection of urine pheromones viachemoreceptors of the lateral antennular flagellum. The specific sensory pathway mediating this recognition is not known. Most of the chemoreceptor cells of this flagellum are found in the unimodal aesthetasc sensilla and project specifically to the glomeruli of the olfactory lobe in the brain. Additional chemoreceptor cells are located among mechanoreceptor cells in bimodal sensilla, including the guard hairs; they do not project to the olfactory lobe. This neuroanatomy suggested that aesthetascs were essential to all complex chemosensory tasks until it was shown that spiny lobsters Panulirus argus can still perform complex food odor discrimination and localization tasks without aesthetascs. Here, we demonstrate that the aesthetascs of H. americanus contain the chemoreceptors necessary for individual recognition of familiar opponents. In contrast to intact and guard hair-shaved animals, lobsters with aesthetascs removed did not recognize previous opponents as shown by second encounters statistically similar in length and aggression to first-encounter fights. Non-aesthetasc chemosensory pathways were incapable of rescuing opponent recognition. Subsequent lesion of all remaining chemoreceptor cells (by immersion in distilled water) abolished recognition and renewed fighting.