A limnological investigation of Irondequoit Bay, a closed-in area of Lake Ontario near Rochester, N. Y., was made between Aug. 15, 1939 and June 13, 1940. No O2 was present during the summer in the deepest part of the lake (20 m.) below 10 m. depth. O2 again became scarce during the winter in the bottom regions, a phenomenon which together with other factors showed extreme eutrophy. Free CO2 was present in extremely high concns. at the stagnation periods in early fall and late winter. Mean alkalinity was correlated with conductivity; very marked increases toward the bottom in alkalinity were observed in Aug. and March. Mean organic P showed a definite correlation with mean organic matter. Specific conductivity was unusually high (600 units). Macroplankton was very abundant during the year except during a short period in Jan. and Feb. Microplankton was very abundant during the sum-mer and spring but became very scarce during the winter months. Particulate organic matter followed the plankton counts very closely and varied inversely with the transparency. Winter stagnation in Irondequoit Bay is believed to result from pollu-tion entering by way of Irondequoit Creek and from large quantities of decomposing material on the bottom. Pollution in Irondequoit Bay is not believed to be serious enough to do more than increase plankton growth. Stagnation conditions would probably be improved if a fairly deep channel were to be cut from the Bay to Lake Ontario.