The concentration of nine types of phosphate was observed at sixteen sampling sites from upstream to downstream in the Han River on the Korean Peninsula throughout the four seasons in 1978-1979. The concentration of all types of phosphate increased from upstream to downstream, especially in the Seoul City area, in terms of the landscape or population density of the basin. The ratio of particulate organic phosphate to total phosphate was constant from upstream to downstream irrespective of the total phosphate concentration level, suggesting that the plankton biomass increased in proportion to the total phosphate concentration. The ratio of dissolved ortho and hydrolyzable phosphate to dissolved total phosphate increased from several percent in the upstream area to more than 90 percent in the downstream region. Results of phosphate analysis in thirteen tributaries showed that the increasing concentrations of phosphate, particularly ortho and hydrolyzable phosphate from the upstream to downstream, were caused by drainage water from cultivated land in the middle reaches and mainly by domestic and industrial waste water in the downstream areas, which were heavily contaminated by fertilizer or synthetic detergent.