Inhibitory effect of UV on phytoplankton photosynthesis was measured by incubating lake samples in quartz bottles transmitting UV, glass bottles cutting UV-B, and glass bottles wrapped with polyvinylchloride films cutting both UV-A and UV-B. Therefore photosynthetic rate of phytoplankton was measured under three different light conditions of PAR+ (UV-A)+ (UV-B), PAR+ (UV-A), and PAR conditions. UV-A significantly inhibited the photosynthetic rate of phytoplankton and UV-B showed small additional inhibition. Three species of phytoplankton, Microcystis, Melosira, and Chlorella showed similar degree of photoinhibition by UV. Suppression of photosynthesis due to UV-A measured by C-14 uptake method showed similar result with O_2 method. The photosynthesis-irradiance(P-I) curve measured in Lake Biwa was a saturated hyperbolic curve in the absence of UV-A conditions. In contrast, the P-I curve in the presence of UV-A showed significant depression of photosynthetic rate at high light intensity. From the results it is certain that UV-A exerts more impact than UV-B in the photoinhibition of algal photosynthesis. Solar UV-A measured in the Tokyo area, Japan, from 1992 to 1993 was about 2 times higher in summer than in winter. However, the ratio of UV-A to PAR was constant irrespective of sky condition, time of a day, and the season. On sunny days in summer, daily productions at the surface of Lake Suwa and Senzoku pond under the presence of UV-A conditions were inhibited by about 50% compared with the rate in the absence of UV-A conditions.