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eof a planktivorous damselfish, Acanthochromis polyacanthus) and either conspecific skin extracts (which contains a chemical alarm cue) or skin extracts from the freshwater swordtail, Xiphophorus helleri. Swordtails, which are allopatric with A. semipunctatus, lack the goby alarm cue and were used to control for a general response to the damaged skin of any fish. Two days after the conditioning, the gobies were exposed to only the neutral chemical cue. Gobies that had been conditioned to the neutral cue paired with conspecific skin extracts on day 1 responded to the neutral cue alone on day 3 with a typical alarm response, including decreased movement and feeding. Gobies that had been conditioned to the neutral cue paired with swordtail skin extracts did not show an alarm response. These results suggest that A. semipunctatus can learn to recognize a novel chemical cue and associate it with danger after a single simultaneous exposure to a novel cue and a conspecific alarm signal. This mechanism of rapid learning of novel chemical cues may be an important way in which coral reef fish learn to recognize predators. 2004 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. Many prey fish recognize cues emitted from their natural predators and respond by performing appropriate defence behaviour (reviewed by Kats & Dill 1998). However, fish often do not respond with a fright reaction to unfamiliar predators (Smith 1997). Furthermore, individuals from populations that frequently encounter predators typically show stronger antipredator responses than do individuals from populations that rarely experience predators (Gelowitz et al. 1993). In some cases, such behavioural differences between populations may result from genetic differences promoted through differing selection pressures (Gandolfi 1972, cited in Smith 1989). For example, Seghers (1974) found that guppies, Poecilia reticulata, collected from different populations in the wild retained differences in schooling behaviour for up to four gener-ations in captivity. makeup. Studies of freshwater fish have shown tha certain responses to particular predatory fish are no always inherited but need to be learned through exper