The interactions between microalgae were examined using the dominant species of phytoplankton isolated from the brackish lake Nakanoumi as one of the biological factors affecting the transition of algal flora in the hydrosphere. The interactions between 2 species among 7 species of the algae were classified to 5 types called amensalism, disoperation, exploitation, commensalism, and neutralism. Especially, it was strongly suggested that the amensalism or disoperation caused by Cymbella sp. was an allelopathic phenomenon which was attributable to the growth inhibitors secreted from the cells.