Behavioural responses including avoidance behaviour of brown trout, Salmo trutta, juveniles were tested in waters of different pH (4.0, 4.5, 5.0, 5.5, 7.0) without Al additions, and with and without Al (< 10, 70 and 190 μg labile Al l−1) at pH 5.0. Single fish were tested in a tank where control (pH 7.0, < 10 μg labile Al l−1) and the various acid water qualities could be separated, and where the fish could swim freely. Each test lasted for one hour and included a five-min acclimation period in control water, a 15-min mixing period, and an additional 40-min test period after the chemical gradient was established. When Al was added at pH 5.0, the fish showed significant avoidance at the highest Al concentration, and spent on average 32% of the time in the acid, Al-rich side versus 68% in the control side. This avoidance response increased with time during the test period. Brown trout juveniles showed significant attraction to low pH water at pH 4.0 (p < 0.05), pH 4.5 (p < 0.001) and pH 5.0 (p < 0.05) (67, 74 and 68% of the time spent in the acid side, respectively), whereas no attraction/avoidance was demonstrated at pH 5.5. The lack of avoidance in the low pH treatments was associated with reduced activity in the acid side, and may have been caused by a slight elevation of free CO2 within the test apparatus.