Water samples were collected from sampling sites in two river systems, 16 from the Sagami River and 3 from the Sakawa River, in Kanagawa prefecture, and examined for the occurrence ofCryptosporidiumoocysts andGiardiacysts in five different occasions, August and December 1998, August and December 1999 and August 2000. According to the method described in the Provisional Guideline issued by the Japanese Ministry of Health and Welfare, oocysts or cysts were detected as follows.
Each of 10-liter water samples was subjected to pressure filtration with a filter, and residue on the filter was dissolved. Thereafter, the solution was centrifuged utilizing Percoll-sucrose gradient, and the concentrate particles were filtered with a 25-mm-diameter membrane filter. Subsequently, the particles were stained with a specific fluorescent antibody, and the stained particles were observed with a microscope.
Of the 19 sample sites, Cryptosporidiumoocysts andGiardiacysts were detected in 12 and 17 water sampling sites, respectively. As a result, some sites where protozoa was repeatedly detected were found in the rivers from which samples were taken in the present studies, whereby unchanged contamination states were made clear.
Interestingly, some tendencies were found such that a larger number ofCryptosporidiumoocysts was detected in tributaries running through the east side of the Sagami River where many stock raising farms are located, and that a larger number of Giardia cysts was found on the west side of the River in which residential areas are developed.

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