Phytoplankton counts on samples collected in Lake Ontario on September 8-18, 1964, showed that green algae were the dominant plankters and diatoms were of secondary importance. The greatest abundance of phytoplankton was close to shore from Toronto, along the southern shore of the lake, and up the eastern shore to the North Channel. The open waters of Lake Ontario were characterized by low numbers of phytoplankton. The relationships among phytoplankton abundance, bottom fauna distribution, and enrichment are discussed.