Bioassays were conducted with Aqualin, a herbicide manufactured by the Shell Chemical Company, to investigate its possibilities in collecting fishes from lotic waters as well as to determine its relativity to toxicity to five species of fishes. A narrow range of toxicity was found between the largemouth bass, b1uegill, bowfin, mosquitofish, and the fathead minnow. The 24-hour TLm (median tolerance limit) values ranged from 0.062 to 0.183 ppm at 71 º F. Bowfin were the most sensitive fish tested. Aqualin was found to be less toxic than rotenone in short exposures; however, a lethal dose of either was required to effectively repel fishes. It was concluded that there is a narrow range of susceptibility between different species of fishes to Aqualin and that, in no case, was it sufficiently wide to indicate any promise for collecting fishes in lotic waters.