Changes of the mutagenic activity of waters treated with chlorine and chlorine dioxide were examined to estimate the mutagenic activity of drinking of water in distribution systems. Chromosomal aberration test using Chinese hamster lung (CHL) cell was carried out to evaluate mutagenic activity. There were differences in forming and decreasing rate of activity inducing aberrations between treated waters. The observed decreasing rate constant of the activity inducing aberrations of chlorinated water was 1.4 to 1.9 times as large as that of water treated with chlorine dioxide. The mutagenic activity of drinking water treated with chlorine dioxide was estimated to be 70-80% of that of chlorinated drinking water. However, the difference in mutagenic activity would be small when drinking water remains long in distribution systems. There were no disinfection by-products with the same variation tendency of activityinducing aberrations after the disinfection.