Exposure of brook trout to chlorine evoked changes in activity, ventilation, the "coughing" reflex, and at lethal levels a heavy secretion of mucus. Locomotory activity increased initially and was subsequently depressed. Both responses were seen at 0.35 and 0.08 ppm, but the initial increase was not seen at 0.04 or 0.005 ppm. The initial increase lasted 2 h, whilst the depressant response was still continuing after 7 days at the sublethal level of 0.005 ppm.The results are discussed in relation to the different selection responses to different chlorine concentrations shown in rainbow trout by Sprague and Drury (1969).