The biodegradation of fungicide pentachloronitrobenzene (PCNB) in water environment was evaluated. The biodegradability of PCNB was different in each region. However, seasonal and yearly variations in biodegradation of PCNB were not found. The biodegradation of PCNB was rapid in river water polluted by PCNB. Pentachloroaniline (PCA) was formed with the biodegradation of PCNB under anaerobic conditions, but PCA was slightly biodegraded under anaerobic conditions. Furthermore, it was assumed that PCNB degrading bacteria isolated from the river water belonged to Pseudomonas sp., which were capable of biodegrading PCNB to pentachlorothioanisole (PCTA). According to the survey of water qualities at Tsumagoi in Gunma prefecture and at Sugadaira in Nagano prefecture, there were differences in concentration ratios of PCNB and PCA. This observed results were consistent with the results of the PCNB biodegradation test, by which the differences between PCNB biodegradation ratios and PCA formation ratios were observed at both areas.