Bacterial assay which can detect the genotoxicity, induction of error-prone repair genes on DNA was applied to samples which were collected at a night soil treatment plant. The genotoxicity of each sample was measured by using this bacterial assay, called the umu test. Strong genotoxic potency was found in the sediments of raw night soil, that is, human feces. There are some substances which show toxicity to the DNA and are attributable to the baked foods or metabolites in human digestive organs. These potencies could not be removed easily in the biological decomposition process through survey of the treatment plant. It was quite clear that more effective reduction could be achieved by ozonation at the treatment condition of about 1 mg O3/mg C.