The ecology of Conowingo Pond (the lower pond) and the Muddy Run Pumped Storage Pond (the upper pond) on the lower Susquehanna River in southeastern Pennsylvania was studied to determine the effects of operation of the Muddy Run Pumped Storage Station. Nearly all the species of fish present in the lower pond are now found in the upper pond. Extensive sampling in the two ponds indicates no detrimental effects of the station operation on the fishes and fisheries of Conowingo Pond. The creation of the 398.9-hectare upper impoundment serves as an area where biomass of fishes pumped up from Conowingo Pond can be increased and exploited by anglers. Wide fluctuations (up to 15 m in a weekly cycle of operation) in water level limit the reproduction of nest-building fishes in the upper pond. About 6,5 times as many fish larvae were pumped up into the upper pond as left during generation. No reliable quantitative estimates could be obtained on the percentage mortality of fishes passing through the pump-turbines. The mortality of fishes associated with the operation of the pumped storage station appears to be negligible. It is concluded that screens are not needed for the protection of fish at the Muddy Run Station. Even if a satisfactory type of screen could be installed substantial mortality due to “gilling” of fishes would occur. Sampling problems unique to the pumped storage project are discussed.