Phenol, a common pollutant in municipal water supplies, cannot be readily detected in low concentrations until it has reacted with chlorine forming odorous chlorophenols. A method is here presented whereby bluntnose minnows (Hyborhynchus notatus) are used to detect the presence of phenols below the threshold for man (0.01 p.p.m.). Fishes of two aquaria were trained to associate the odor of phenol with food; the odor of a p‐chlorophenol with punishment. The minnows of two other aquaria were trained to the same two odors, but with reverse meanings. Upon completion of the training period, the minnows were discriminating successfully between these two substances at concentrations of at most 0.0005 p.p.m. A generalization test, using p‐chlorophenol as the third odor, indicated that the fishes were able to distinguish it from the two training odors. The biological assay of phenolic compounds by this method is primarily one of detection and not of quantitative analysis.