The community of phytoplankton and zooplankton and the environmental factors were investigated in Lake Soyang in spring bloom season from January to July 1994. Phytoplankton dominants in Lake Soyang were Anabaena spp., Microcystis aeruginosa, Asterionella formosa, Asterionella gracillima, Melosira distans, Synedra acus, and Asterococcus limneticus. Zooplankton dominants were Polyarthra spp., Keratella spp., Asplanchna placentula, Bosmina coregoni and Daphnia longispina. Phytoplankton and zooplankton were clearly related each other with respect to biovolume, not to numbers. Microcystis aeruginosa rapidly increased and Daphnia longirostris disappeared in July, possibly because Microcystis aeruginosa excrete toxic substances to Daphnia longirostris. Transparency gradually decreased from January to June, but increased in July. The highest cell density of phytoplankton was observed in April, and one month later the zooplankton density reached a maximal level, implicating that spring bloom of photoplankton provided good feeding condition for zooplankton.