The benthic communities in the Oh-kawa river with non-point pollution source were investigated in April and June, 1992, in order to evaluate aquatic environmental conditions. The water quality of the Oh-kawa river was characterized by the low BOD concentration and high concentrations of nutrients such as nitrate nitrogen and phosphorus.<br>The total recognized species of benthic animals were 56 and 71 in April and June respectively. The dominant species was Hydropsyche ulmeri in the upper reaches of the Oh-kawa river and Antocha sp. in the lower reaches.<br>The diversity index on the basis of dry weight was more significant than that on the basis of individual number in examining the structural change of the benthic animal community. The similarity index could be used to judge not only the change of environmental conditions at each sampling station but the stability of the benthic animal community. The pollution index was nearly correlated with water quality parameters such as BOD and so on, but would become the index for the decision of the water quality rank.<br>Percentages of number and standing crops for Antocha sp. were expressed by the function of NO₃-N or PO₄-P concentration respectively.