Invertebrate population changes were traced during 1966–1968 in Emmaline Lake, Colorado, after removal in 1964 of a population of stunted brook trout. Species composition of the zooplankton changed greatly after fish removal; in the absence of fish predation, large species were dominant. Populations of mud dwelling midges increased after fish removal; in the absence of fish predation, population regulation appeared to occur through density-dependent larval mortality. Insect emergence over rocky areas was highest in the second summer after fish removal, following increases in periphyton production. In 1968, the lake was restocked with yearling cutthroat trout that fed primarily on midge pupae and Daphnia middendorffiana, and doubled in weight during the first summer in the lake. Growth efficiency of the trout was estimated to be 10–30%. Less than half of the daily ration was present in trout stomachs at any time, as judged from observed growth rate and potential growth efficiency. A computer simulation model of the fish production system suggested that yield and optimum management policy (stocking rate and harvest rate) are most strongly affected by the quantities of terrestrial insects available to fish. Optimum stocking rate is apparently much lower than that which would maximize total invertebrate yield to fish.