Three kinds of phytoplankton were cultivated, and the contribution of dissolved organic matter (DOM) released from the phytoplankton was examined to clarify the cause of organic pollution of Lake Biwa. Microcystis aeruginosa, Staurastrum dorcidentiferum, and Cryptomonas ovata were evaluated with regard to cultivation. A significant peak (M(w): <3000 Da) was mainly detected in the algal DOM released from plankton during cultivation by gel chromatography with a fluorescence detector (E(x) = 340 nm, E(m) = 435 nm). Since this peak corresponds to a peak with lower molecular weight in three peaks detected in the surface water of Lake Biwa, it can be concluded that the algal DOM released from the plankton during cultivation makes a considerable contribution to the refractory organic matter in Lake Biwa. Three fluorescence maxima were observed in the cultivation of three kinds of phytoplankton: two fulvic-like fluorescence peaks (A and B) and a protein-like fluorescence peak (C). These peaks became larger as their cell counts of plankton increased. As for the fractionations of algal DOM using DAX-8, the ratio of hydrophilic DOM is fairly high in DOM produced by three kinds of phytoplankton. The order of the amount of algal DOM per cell volume during cultivation was Cryptomonas ovata > Microcystis aeruginosa > Staurastrum dorcidentiferum. These results suggest that the increase of the refractory organic matter in Lake Biwa may be attributed to a change of the predominant phytoplankton.