Using a series of field enclosure experiments, we investigated the trophic dynamics of Leptodora kindtii in Lake Mendota. Leptodora fed on both small and large Daphnia as well as on Diaphanosoma, Conochilus, and copepod nauplii. These results were combined with field data and a computer simulation model to examine how various densities of Leptodora might affect the densities of Daphnia and other zooplankton in Lake Mendota. Leptodora could account for all of the mortality calculated for Daphnia galeata mendotae in late June to early July. Reduced numbers minimized Leptodora’s potential, however, for direct control of Daphnia earlier in the year. The simulation model demonstrated that, because the dominant planktivorous fish in the lake fed on both Leptodora and daphnids, a decrease in planktivorous fish could result in higher Daphnia mortalities due to increased predation by Leptodora.