The Misato Purification Plant, Waterworks Bureau of Tokyo Metropolitan Government, in 1992 encountered an off-flavor problem due to the formation of 2-methylisoborneol (MIB) in a small tributary. This tributary had become polluted by rapid urbanization of the basin. However, it is situated at a point lower than the Plant and does not discharge directly into the main river. MIB produced in the downstream tributary was transported by a back current and via a Conveyance Pipe to the main river. The causative blue-green algae in the tributary water was determined through chemical and biological analysis of field samples as well as cultured materials. MIB production by one species of Oscillatoria isolated from the field samples was demonstrated. The strong MIB production potential of this species was also shown by a comparison of MIB production per unit length of trichome (16 pg/500μm) between this and other species reported previously. It was found that the presence of only one filament of this species in 1 ml of water could result in MIB levels above the threshold concentration of MIB for Japanese people. Results of this study revealed the importance of monitoring causative organisms and hydraulic conditions in tributary rivers.