During June and July 1970, an estimated 5.3 million eggs (4.1/103m3) and 56.6 million young fish (43.8/103m3), 3–62 mm FL, were pumped by the Muddy Run Pumped Storage Generating Station from Conowingo Reservoir (lower Susquehanna River, Pennsylvania) into Muddy Run Pumped Storage Reservoir. A maximum of only 2% of the eggs and 39% of the young were returned to the lower reservoir during generation. The eggs were predominately Carpiodes cyprinus. Of the young, 73% entrained during pumping and 83% entrained during generating were Ictalurus punctatus, Cyprinus carpio, and Carpiodes cyprinus. Similar results are reported for 1969. The species and numbers of specimens entrained reflect the utilization of the area near and above the lower reservoir intake discharge structure as spawning and nursery grounds, the extent and peaks of spawning seasons, and the phases of development and behavior of the young. Sufficient numbers of specimens pumped into and retained by the upper reservoir, in which natural reproduction is nearly precluded by the extensive fluctuation of water level, survive to maintain many of the reservoir’s fish populations.