A quantitative survey was performed to understand the annual and diurnal profiles of Cryptosporidium and Giardia, representative waterborne infectious protozoans, in river water which is used for drinking water sources in Japan. To investigate the annual profiles, 84 river water samples were collected at 7 sites in the tributary rivers of the Tone River basin in Japan from June 2008 to February 2010. Cryptosporidium and Giardia were detected in 59 (70%) and 64 (76%) out of the 84 samples (10

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... ers each), showing the highest concentration of 344 oocysts/10 L and 144 cysts/10 L, respectively. Annual variation of the concentrations of Cryptosporidium and Giardia was high. The ratio of the maximum concentration to the mean value at each sampling site ranged from 2-8 except for one sampling site in which the frequency of detection was extremely low. To investigate diurnal profiles, 15 river water samples were collected at 3 sites in the tributary rivers of the Tone River on October 9 th , 2008. The maximum concentrations of Cryptosporidium and Giardia in some sampling sites were approximately 10-fold higher than the lowest value. The correlation between the anaerobic spore-forming bacteria and these infectious protozoans was stronger than other microbial indicators (total coliforms, Escherichia coli and heterotrophic bacteria).