Daphnids were the only zooplankton consumed by rainbow trout in both lakes and by yellow perch in the combination lake even though there were many other genera of zooplankters. Both species were very size selective and usually consumed only Daphnia over 1.3 mm in size while ignoring the many and often more numerous smaller zooplankton. Despite the broad range in size of these trout (7.9 to 17.2 inches) and yellow perch (2.8 to 9.8 inches), there was no strong evidence of an association between their length and the size of the Daphnia they consumed. Introduction of rainbow trout in the combination lake had no apparent effect on the daphnid population. However, changes in the net plankton did occur in the trout lake after the introduction of rainbow trout, smelt, and fathead minnows. Most obvious changes were: (1) the complete elimination of D. pulex and subsequent replacement by two other smaller species within 4 years, (2) a decrease in the average size of the daphnids from 1.4 mm to 0.8 mm, (3)a decrease in the percentage of daphnids larger than 1.3 mm from an average of 53.8% to 4.7%, and (4) a reduction in the volume and percentage of daphnids comprising the net plankton even though there was no reduction in their numbers. The gap left by the elimination of the large Daphnia was filled by other smaller zooplankters. Effects of predation on Daphnia, as well as the interactions between daphnids and planktivorous fishes are discussed. This study emphasizes the importance of determining the abundance of zooplankton of the proper sizes in the plankton when the survival of planktivorous fishes in lakes is investigated. This report is part of a comprehensive investigation of the relationship of food to the survival of rainbow trout (Salmo gairdneri) planted in Michigan lakes. Examination of the stomach contents of the rainbows has indicated that Daphnia was the principal form of plankton being utilized. The objective of this study was to determine the size of daphnids being consumed, and the effect, if any, fish predation might have upon the size composition of the Daphnia population. Many investigations have beer concerned with the density or biomass of daphnids and have assumed that numbers or mass were the important indices of their availability to fish. Little consideration has been given to the size of Daphnia comprising the plankton or to those consumed by fish. Hall (1964) observed that adult ciscoes (Leucichthys artedi) and black crappies (Pomoxis nigromaculatus) fed extensively on the large Daphnia pulex present in early spring but did not eat the smaller adults of Daphnia galeata mendota which also were present. Gerking (1962) noted that bluegills (Lepomis macrochirus) selected the larger Daphnia. He compared the size distribution of Daphnia from net plankton catches with the size distribution of Daphnia from bluegill stomachs. Data were published recently which indicated size-specific predation of fish upon zooplankton populations (cf. Brooks and Dodson, 1965). This study indicated that alewives (Alosa pseudoharengus and A. aestivalis) eliminated the Daphnia in some Connecticut lakes. Brooks and Dodson observed changes in the size distribution of daphnids in the net plankton after alewives were introduced into one of the lakes but did not examine stomachs to determine the size of the cladocerans consumed by the alewives. In this paper, data are presented on the size-frequency distribution of Daphnia found within stomachs of rainbow trout and yellow perch (Perca flavescens, and on the relationship of the size frequency of Daphnia in the plankton to that in fish stomachs.