We measured the mutagen formation potential (MFP) of 42 river water samples from 37 rivers throughout Japan using the Ames mutagenicity assay, to investigate the nationwide level of mutagen precursors contained in river water. The MFP of river water was highest when Salmonella typhimurium strain TA100 was used without metabolic activation. The MFPs ranged from 210 to 20,200 net rev. · l^-1 and the median was 2,520 net rev. · l^-1, which shows that the values varied greatly between samples. About 60% of the samples showed MFPs in the range from 1,500 to 3,500 net rev. · l^-1. Dissolved organic carbon (DOC), E₂₆₀, NH₄-N, NO₂-N, NO₃-N and T-N concentrations were also measured. A strong positive relationship was observed between logarithmically-transformed DOC and MFP. However, the MFP/DOC ratio varied widely from 110 to 2,160 net rev. · mg^-1. We also investigated the relationship between MFP and river basin characteristics such as population density. The results suggest that mutagen precursors are strongly influenced by human activities. In addition, rivers were classified using a cluster analysis of water qualities (MFP, DOC, E₂₆₀, NH₄-N and NO₂-N+NO₃-N).