We compared the behaviour of wild and captive-bred butterfly splitfins (Ameca splendens), an endangered freshwater fish, to investigate whether captive breeding results in the behavioural divergence of wild and captive individuals. In a first experiment, we examined whether the captive environment allows for the similar expression of behaviours observed in wild fish. The foraging, courtship and aggressive behaviour of fish in their natural habitat (in Mexico) was compared with that of their counterparts that have been bred at London Zoo, UK, for 40 years. These in situ observations revealed that wild fish were preoccupied with searching for food whereas captive fish engaged more in aggressive interactions. In a subsequent laboratory experiment we compared the behaviour of wild-caught and captive-bred fish under standard conditions in two novel habitats: structured (enriched) and unstructured (bare) aquaria. Overall, captive-bred butterfly splitfins displayed higher levels of aggression than wild-caught fish. The relationship between aggression and habitat structure was influenced by density; captive-bred males were more aggressive when observed in structured habitats than unstructured ones, but only when they were stocked at a high density. We also found an effect of tank structure on foraging behaviour, with individuals spending more time foraging in unstructured tanks than structured tanks. There was no effect of captive breeding or habitat structure on courtship behaviour.Our findings suggest that captive environments can promote the development of aggressive behaviour which may affect the suitability of captive-bred fishes for reintroduction into the wild. (c) 2006 Elsevier Ltd. All rights reserved.