D. aruanus retreat into their home site (coral) on the approach of a predator or unfamiliar object. This escape response occupies time that could otherwise be spent on other activities and responses to inappropriate stimuli may penalize D. aruanus. The ability of D. aruanus to discriminate various species of fish towards which responses and no response should be made was investigated in the field. Five predatory and five non-predatory species of fish were presented to social groups at either of two presentation distances. D. aruanus exhibit an ability to discriminate the two categories of fish and react to the predatory species by remaining closer to or within the protection of their home site. These effects were more marked at the closer presentation distance. D. aruanus did not react significantly in these respects towards species of non-predatory fish. Interspecific aggressive behaviour is described and was exhibited towards species of predatory fish only. The function of this behaviour is briefly discussed.