Using a channelled avoidance trough, immediate behavioral reactions of blacknose dace to a domestic sewage effluent and its toxic constituents, chloramines, free chlorine, and ammonia, were investigated. Two avoidance indexes were computed from time spent and number of entries by the fish into a test solution channel and a clean water channel. The time avoidance index was found to be a more sensitive measurement than the entry avoidance index for fish behavioral reactions. On the basis of the time avoidance index, the fish did not avoid unchlorinated sewage effluent and ammonium chloride solution in concentrations as high as 270 mg/liter as ammonia-nitrogen. The fish did avoid chlorinated sewage effluent, chloramines, and free chlorine. The degree of fish avoidance increased as total chlorine concentration increased. The threshold avoidance concentration under the test conditions was 0.13 mg/liter as total chlorine for chlorinated sewage effluent, 0.18 mg/liter for chloramines, and 0.61 mg/liter for free chlorine (DPD colorimetric method). The degree of fish avoidance and the pattern of behavioral responses to chlorinated sewage effluent were almost identical with those to chloramines, but different from those to free chlorine. The results suggest that chloramines are the major factor causing avoidance reaction of blacknose dace to chlorinated sewage effluent.