Introduction -- Yellowstone Lake fishery -- Yellowstone River fishery -- Summary -- Literature cited -- Appendix A, Distribution of fishing -- Appendix B, Maturity of fish in catch -- Appendix C, Growth increments -- Appendix D, Year-class strength -- Appendix E, Catch curves. Equilibrium yield of the cutthroat trout, Salmo clarki lewisi Girard, in Yellowstone Lake, Wyo., is determined from data on catch and spawning runs from 1945 to 1961. Changes in growth rate, spawning runs, mortality rates, and year-class strength are related to differences in total catch. Three stages of exploitation of the stock are defined and the maximum safe catch or equilibrium yield is estimated at 325,000 trout. Management of the sport fishery according to equilibrium yield is discussed with reference to regulations, distribution of fishing pressure, planting, and interspecific competition. The Yellowstone River fishery is treated briefly.