Lyon County Lake was drained during the early fall, 1962, as the initial step in a program to rehabilitate its fishery. Beginning in March, 1963, and continuing through June, 1967, variations in physicochemical conditions, primary productivity, and the relative abundance and composition of zooplankton were recorded for the purpose of determining changes in these features during and immediately following reflooding. The annual mean dissolved oxygen, phosphate, specific conductance and gross primary productivity during the final year of the study were significantly lower than previously recorded values. Twenty species of Cladocera and fifteen species of Copepoda have appeared in townet samples. In general, density and biomass decreased throughout the study. Initially, Cladocera were approximately twice as abundant as the Copepoda; however, by the final year of the study, their densities were approximately equal but Copepoda biomass was nearly double that of the Cladocera. Lyon County Lake, approximately 19 km northeast of Emporia, Kansas, is located in the Missouri River Basin which drains the northern half of the state. The lake drains an area of 583 ha which is predominantly grassland of the Andropogon scoparius, A. gerardi, Sorghastrum nutans, Panicum virgatum, and Bouteloua curtipendula association. It was constructed for the Kansas Forestry, Fish and Game Commission in 1934 by the Civilian Conservation Corps and first reached capacity in 1937. At capacity level, the lake had a 54.7 ha surface and a maximum depth of 13 m. Sedimentation has since reduced the maximum depth to less than 12 m. The lake is approximately 1200 m long with a maximum breadth of about 500 m; its long-axis is oriented in a southwesterly to northeasterly direction. Lyon County Lake maintained a relatively high fish production for nearly twenty years, but by 1962 fishing success had so declined that the State Fisheries Division initiated a program to rehabilitate the lake's fisheries. Drawdown was commenced during the 1962 summer; and by October the impoundment was reduced to a small, shallow pool which was first seined and then treated with fish toxicant to destroy the remaining fishes. During Februray and March, 1963, the lake was restocked with Channel Catfish (Ictalurus punctatus), Largemouth