Arginine vasotocin (AVT) and the homologous arginine vasopressin (AVP) neuropeptides are involved in the control of aggression, spacing behaviour and mating systems in vertebrates, but the function of AVT in the regulation of social behaviour among closely-related fish species needs further clarification. We used immunocytochemical techniques to test whether AVT neurones show species, sex or seasonal differences in two sympatric butterflyfish sister species: the territorial monogamous multiband butterflyfish, Chaetodon multicinctus, and the shoaling polygamous milletseed butterflyfish, Chaetodon miliaris. The territorial species had larger AVT-immunoreactive (-ir) somata within the preoptic area, and higher AVT fibre densities within but not limited to the ventral telencephalon, medial and dorsal nucleus of the dorsal telencephalon, torus semicircularis, and tectum compared to the shoaling nonterritorial species. Furthermore, AVT-ir somata size and number did not differ among sexes or spawning periods in the territorial species, and showed only limited variation within the shoaling species. The distinct difference in AVT neuronal characteristics among species is likely to be independent of body size differences, and the lack of sex and seasonal variability is consistent with their divergent but stable social and mating systems. These phenotypic differences among species may be related to the influence of AVT on social spacing, aggression or monogamy, as reported for other fish, avian and mammalian models. The present study provides the first evidence for variation in vasotocin neural organisation in two congeneric and sympatric fish species with different social systems.