Bythotrephes cederstroemi Schoedler (Cercopagidae: Cladocera), a freshwater predator, invaded the Laurentian Great Lakes from 1984 to 1988. Previously observed only in Europe, the U.S.S.R. and China, these are the first records of the genus in North America. Bythotrephes is a large zooplankter that reaches an average of 10 mm in length. As an adult, its body size is only 2 mm in length, but it has an unusually long caudal appendage that exceeds the body by 3 to 4 times. Bythotrephes can directly influence the Great Lakes fish community in two ways. As a large zooplankton, it can represent a food item to both large and small fish, and, as a predator on zooplankton, it can compete with small planktivorous fish for resources. The purpose of this research is to address whether Bythotrephes is a potential prey item to small planktivorous fish given its long rigid caudal spine. In three chapters, I explore the possibility that the spine of Bythotrephes acts as a defense against small vertebrate predators, I test its value as a deterrent to juvenile fish, and finall I determine fish behavioral response to Bythotrephes and alternative prey over time. This thesis provides the first evidence of a freshwater zooplankter having a post-contact defense effective against young fish. I demonstrate that the caudal appendage operates as a mechanical deterrent to ingestion, and I illustrate the decision-making process by which fish learn aversion to Bythotrephes. The large size and conspicuity of Bythotrephes, coupled with the ingestion difficulty the spine provides, induces behavioral changes in fish that range from preference, to rejection and recapture, to aversion. Juvenile fish reduce their predation rate on Bythotrephes, but maintain interest. Fish become distracted by Bythotrephes and forage less on alternative prey. This overall reduction in fish predation rate can benefit Bythotrephes in two ways. There is a decrease in predation risk to itself, but also a decrease predation risk to other zooplankton such as Daphnia, a potential food resource.