The test fish in this study demonstrated levels of avoidance that differed widely among the organic herbicides tested. The underlying mechanisms involved in distinguishing between different organic compounds are unclear, but are possibly related to the ability of the olfactory system to discriminate between certain side chains in complex organic molecules. In general, the results of these studies suggest that rainbow trout would not be killed by, and would not avoid, relatively low concentrations of dalapon, 2,4-D (DMA), glyphosate, Aquathol K, diquat, or TCA. However, trout present in acrolein-treated water probably would not avoid a lethal exposure.