Safety of herbicide CNP during aerobic biological treatment. was evaluated. Batch biodegradation experiments with bacteria which was accumulated from Lake Biwa water were carried out. It was found that these bacteria decomposed CNP releasing all chlorine atoms of CNP in the culturemedium within 96 hours of cultivation. A by-product of CNP, C₆H₂Cl₃-O-C₆H₅ was found by mass spectrum analysis with gas chromatograph. Chromosomal aberration test using Chinese hamster lung (CHL) cell was carried out to evaluate mutagenic activity of aerobic biological treated water including CNP. It was found that thecontribution of CNP to activity inducing aberrations of biological treated water decreased. On the other hand, the contribution of the by-products of CNP was increased. As a result, activity inducing aberrations of treated waterdid not reach the control test level.