Preference responses of zebrafish to 10−3, 10−4 and 10−5M alanine (Ala) were concentration- dependent. Behavioural responses to copper (Cu) and Cu + Ala mixtures were also assessed. Zebrafish avoided 100 and 10 μg Cu l−1, but not 1 μg l−1. Mixtures of 10−3 m Ala+ 100 μg Cu l−1 and 10 4 M Ala + 10 μg Cu 1−1 were avoided as intensely as was Cu alone. Responses to 10−3 M Ala + 10 or 1 μg Cu l−1 and 10 4 M Ala +1 μg Cu l−1 did not differ statistically from controls (no detectable preference or avoidance). These results demonstrate, firstly, that a concentration of a pollutant avoided by itself (10 μg Cu l−1) may not be avoided when encountered with an attractant chemical stimulus (Ala) and may suppress the preference for an attractant stimulus, and secondly, that a concentration of a pollutant not avoided by itself and not considered deleterious (1 μg Cu l−1) suppresses attraction to Ala (an important constituent of prey odours for many fishes).