SUMMARY1 Experiments were designed to determine the relative importance of chemical and visual stimuli in eliciting predator avoidance behaviour in juvenile freshwater crayfishPacifastacus leniusculus(Dana).2 Crayfish placed in visual and/or chemical contact with one of two predators exhibited marked avoidance behaviour, spending less time walking and climbing and more time within shelters.3 The combined effects of both visual and chemical stimuli increased crayfish shelter use and reduced walking and climbing activity to a greater degree than either stimulus when presented alone.4 Crayfish exhibited avoidance behaviour in response to chemical stimuli during periods of light and darkness. Visual detection of predators elicited avoidance behaviour during the day.5 It is suggested that the behavioural response of P.leniusculusto chemical stimuli reduces the likelihood of being detected by visual predators, and that chemical stimuli lower the response threshold for avoidance behaviour in crayfish reacting to visual stimuli. The adaptiviry of using chemical cues to detect predators is e