Genotoxic activities have been detected in the river water. The umu test, which can detect DNA damage, was carried out with blue rayon extracts of various water samples from the river systems and treatment plants. HPLC fractionations followed by MS or UV analysis were applied to the extracts to identify the genotoxic substances. Trp-P-2 (3-amino-1-methyl-5H-pyrido[4,3-b]indole), which is one of mutagenic and carcinogenic heterocyclic amines, was detected in concentration from 1 μg·l^-1 to 1 ng·l^-1. These results show that carcinogenic substances related to humans' daily diet remain after usual biological treatment. To evaluate the level of Trp-P-2, the added risks were calculated by use of the doseresponse relationships obtained in a carcinogenic animal experiment with mice. The values at assumed exposures of 2l per day of samples from the night soil treatment plant over an average life span of 70 years for 70kg person were in excess of 10^-6. Because of the possibility of the reuse of wastewater, the need for the investigation on the safety of the secondary effluent from the treatment plants was suggested.