SUMMARYOnondaga Lake, possibly the most polluted lake in New York State, supports an algal flora characteristic of a eutrophic lake with an admixture of saline species. Floral diversity is noteworthy under the circumstances. Seasons in the lake may be identified by the succession of flora. High salinity of lake water results in layering at the outfall (as in an estuary, in reverse) such that river water may enter the lake at the outlet despite continuous net outflow. Certain species of algae appeared first at the outflow station and then spread through the lake. Phosphorus and nitrogen are never limiting, although seasonal diminution in the latter may influence the composition of the flora. Silica diminution was related to blooms of diatoms and may then be limiting for further growth of diatom populations. No obvious controlling relationship exists between herbivores and phytoplankton populations. The euphotic layer is typically much shallower than the thermocline. Therefore turbulence and mixing may play an important role in controlling certain populations. A brief midsummer clearing in 1969, characteristic for the lake, resulted in serious oxygen depletion and cannot be accounted for with the available data. Among metals, chromium and copper are high in the lake as the result of industrial discharges. The former may be inhibiting, the latter probably is inhibiting to algal growth. Consequently, measures underway to “clean up” the lake may result in increased blooms. Biomass calculations have been made for the major phytoplankters, and a list of 82 species is presen