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Behaviour of chaetodontid fishes relevant to Lorenz's “poster colouration” hypothesis has been studied. Field observations of 20 species at Lizard Island, Great Barrier Reef, showed that these chaetodontids rove over large areas of reef, occurring singly or in pairs and sometimes sizeable groups. The behaviour of chaetodontids does not support the supposed explanation of poster colours as intra-specific sign stimuli serving to space individuals. During daytime, agonistic encounters are rare, even amongst groups, and for most species there is no evidence of territoriality. Experiments presenting painted and blank chaetodontid models to eight species generally confirmed these observations. At dusk and dawn, chaetodontids are aggressive and defend regular night-resting places. Agonistic encounters though, are both intra- and inter-specific; they serve to space individuals over only very small distances, and colouration seems to have little warning value. The question of the functional significance of poster colours remains largely unsolved. Amongst chaetodontids, territoriality clearly is not a major function and the evidence suggests that other kinds of social communication, as well as predator avoidance, have probably been important in the evolution of poster colouration.