The influence of natural phytoplankton cell classes upon the response of urea decomposition was investigated in four reservoirs - Lakes Paro, Chuncheon, Soyang and Uiam - in the North Han River System. The decomposition rate of urea was 0.3 to 29.4 μ㏖ urea m^(-3) · hr.^(-1) in the light and 0.2 to 14.9 μ㏖ urea · m^(-3)·hr.^(-1) in the dark. Much higher decomposition rates were observed at the eutrophic stations in Lake Uiam. The urea decomposition in the smaller fraction (<5 ㎛) exceeded that in the middle (5∼25 ㎛) and the larger fraction (>25㎛). No differences in the ratios of urea decomposition to chlorophyll-a or photosynthesis among three fractions were observed. These trends were no different between sampling areas and reservoirs. This may be mainly due to the difference in the standing crop of phytoplankton. The greater part of urea decomposition was the phase of the CO₂ liberation rate into water. 8 to 50% of the urea decomposition was incorporated into the particulate phase in the light, but this was much lower in the dark. A close relationship between urea decomposition and chlorophyll-a or photosynthesis was observed in the three fractions, which have high correlation coefficients. The results of the present study indicate that urea in reservoirs decomposes by phytoplankton rather than bacteria, and thus phytoplankton is more competitive than bacteria.