It has been my intent to investigate the processes which regulate aquatic community composition by discovering those factors which influence the distribution and abundance of one invertebrate predator, Leptodora kindti (Cladocera). By constructing predictive models using published survey data for North American lakes, I suggest that biological factors are important in determining the distribution and abundance of Leptodora. Three distinct patterns emerged from my analysis: 1) Leptodora and the midge larva Chaoborus americanus have non-overlapping distributions; 2) Leptodora is found exclusively in lakes inhabited by planktivorous fish; and 3) Leptodora occurs most frequently in large lakes populated by Daphnia species. I have conducted laboratory and field experiments to explain why Leptodora never co-occurs with Chaoborus americanus. Results from one-on-one encounter trials, seven-day aquarium experiments, and two-week in situ enclosure experiments indicate that third and fourth instar C. americanus are able to consume juvenile Leptodora and significantly reduce its population size. Furthermore, my demonstration that planktivorous fish differentially select midge larvae over Leptodora in laboratory choice experiments suggests that planktivorous fish are able to prevent the establishment of sizable C. americanus populations and release Leptodora from its deleterious interaction with the phantom midge. Alternatively, Leptodora may be more successful in the presence of fish than in the absence of fish because size-selective vertebrate predation produces a zooplankton community which is preferred by Leptodora. By monitoring the zooplankton in 750 liter enclosures during a 16-day field experiment, I was able to show that the bluegill sunfish (Lepomis macrochirus) is able to skew the zooplankton community toward small-bodied individuals which are suitable prey for Leptodora. When in contact with the fish, however, few Leptodora survived to take advantage of the newly available food. Finally, stochastic processes alone may explain the distribution of Leptodora among lakes inhabited by fish. The introduction of Leptodora into a local lake, whose zooplankton community has been monitored for over twenty years, suggests that even short inter-lake distances are effective barriers to the dispersal of some zooplankton species.