Using XAD-2 resin and Ames' assay, mutagenicity of tap waters and the River Shinano waters in Niigata Prefecture of Japan were measured during the winter period from 1988 to 1990 with the test strain of TA100.<br>Without S-9, maximum, minimum and average mutagenicity for 26 tap waters were 133 ngl^-1, N. D. and 31 ngl^-1 as 4NQO. In contrast, with the addition of S-9, mutagenicity were reduced by the average of 91.5%. Mutagenicity of tap waters gained by the rapid sand filtration of river water were much stronger than those by the slow sand filtration of river water and/or the direct supply of groundwater.<br>On the other hand, maximum, minimum and average mutagenicity for the 9 River Shinano waters without S-9 were 18 ngl^-1, 5 ngl^-1 and 13 ngl^-1 in terms of 4NQO. Mutagen intensity was reduced by the average of 28.4% with the addition of S-9. High correlation coefficients were found between the river water mutagenicity and "permanganate value, ammonia, total phosphorus and/or BOD". It is understood that the mutagenicity of the River Shinano was closely related to the organic matter which comes from ordinary human life and animal excretion in the basin.