Concentrations of 137Cs, stable cesium and potassium, and the intake and elimination rates of these nuclides were determined in bluegill and 6 other species of fish inhabiting a small radioactively contaminated lake in Oak Ridge, Tennessee during the period from June 1967 to January 1969. Cesium-137 concentrations increased by a factor of 4 in bluegills (Lepomis macrochirus Raf.) from 1 to 70 g, hut the concentration of stable cesium decreased slightly with increasing fish size. In all species the concentrations of 137Cs cycled seasonally with a minimum in summer and a maximum in winter. Radiocesium concentration was highest in species feeding on food from which a high percentage of 137Cs was absorbed. The concentration of stable cesium cycled similar to that of 137Cs. Concentrations of potassium decreased somewhat with increasing fish size, but did not fluctuate seasonally. There were only small differences in the concentration of potassium in different species. The specific activities of 137Cs in fish were in the same ranges as the specific activity of 137Cs in water. In bluegill the absorbed 137Cs was eliminated by a two-component exponential process. At I5°C, the rates were 0.0912/day (fast component) and 0.00370/day (slow component) in bluegill above 70 g. The proportion of the body burden eliminated by the fast component was 37 % after a single feeding of food contaminated with 137Cs; in the equilibrium state, the proportion of the body burden eliminated by the fast component was only 2.3 %. Rates of cesium elimination decreased with the increasing weight of bluegill. There were no significant differences in rates of the slow component between fish in equilibrium with 137Cs intake and in fish that had received a single feeding of 137Cs. Potassium was eliminated at a single exponential rate, 0.019/day at 15°C in bluegill. The absorption of potassium was almost complete for all types of food. Intake rates of 137Cs, stable cesium, and potassium in bluegill were calculated on the basis of seasonal changes in body burden and elimination rates of these nuclides.