Hosts may use two different strategies to ameliorate negative effects of pathogen attack: resistance (the ability to limit parasite burden) or tolerance (the ability to limit damage). There is increasing recognition that these two strategies may be interrelated, and influence the epidemiology of disease and host-pathogen co-evolution. Despite these potential impacts tolerance is rarely considered. Our long term goals are to identify mechanisms of pathogen tolerance and resistance in snail vectors of schistosome parasites and to determine their epidemiological and evolutionary dynamics so that vector host defense mechanisms may be exploited to reduce infections in humans. Our objectives here are to characterize tolerance and resistance in a model system of schistosomes and snails and also in a snail species taken directly from transmission sites in Kenya. Our central hypothesis is that tolerance and resistance traits vary in populations of snails and are negatively associated. This contribution is significant because it will be the first step in a continuum of research aimed at determining the role of pathogen tolerance in host defense and how this strategy influences pathogen transmission and host-parasite evolution. Furthermore, tolerance mechanisms represent potential targets for control of schistosomiasis an important disease of humans. Data and resources generated from this work will support future proposals aimed at determining 1). how tolerance mechanisms influence pathogen transmission, 2). how tolerance and resistance mechanisms evolve in populations and in response to environmental change, and 3). the mechanisms of tolerance and resistance in order to assess them as targets for schistosomiasis control.