Phenotypic plasticity is an important means through which animals respond to unpredictability and stressors in the environment (Fordyce, 2006;Schlichting and Pigliucci, 1998). Plasticity can occur at many levels, from physiology and morphology to behavior and life history (King and Murphy, 1985). Organ morphology, for example, can be highly flexible in many animals (Piersma and Lindström, 1997;Starck, 1999). The size of gastrointestinal (GI) tract organs changes with energetic and functional demands, increasing during lactation and cold exposure in rodents, decreasing in preparation for migration in birds, and changing over active and hibernation seasons in lizards (e.g. Hammond et al., 1994;Piersma and Lindström, 1997;Tracy and Diamond, 2005). Similarly, metabolic rates can change with season and ambient temperature (e.g. Hill, 1983).Parasites represent an important stressor for many animals, and their effects on host phenotypes can include direct pathologies and host plasticity. For example, parasitism can cause liver and intestinal damage, anemia and increased thermal conductance (Booth et al., 1993;Holmes and Zohar, 1990;Meagher, 1998;Schall et al., 1982;Tocque, 1993;Wiger, 1977). Parasitism can also lead to changes in the energetics and performance of animals, including reduced feeding and activity, impaired anti-predator and competitive behavior, and altered metabolic rates (Arneberg et al., 1996;Booth et al., 1993;Cunningham et al., 1994;Freeland, 1981;Poirier et al., 1995;Rau, 1983a;Rau, 1983b;Rau and Putter, 1984;Schall et al., 1982;Symons, 1985). Finally, parasitized animals may show shifts in organ size and body tissue composition (Kristan, 2002;Kristan and Hammond, 2000;Kristan and Hammond, 2001;Tocque, 1993). Although it is difficult to distinguish between direct parasite impact and host compensation, host alterations in some systems appear to represent phenotypic plasticity to minimize the fitness costs of infection (e.g. Booth et al., 1993;Kristan and Hammond, 2001;Podesta and Mettrick, 1976). Indeed, parasitism can have strong, negative impacts on host fitness through decreases in survival and reproduction (e.g. Crews and Yoshino, 1989;Fuller and Blaustein, 1996;Goater and Ward, 1992;Neuhaus, 2003;Zuk, 1987). The host-parasite relationship, therefore, provides an interesting and ecologically important stage on which to examine phenotypic plasticity and fitness consequences of stressors. Additionally, understanding the phenotypic response of individual hosts is imperative for understanding populationlevel effects and coevolution between hosts and parasites Animals routinely encounter environmental stressors and may employ phenotypic plasticity to compensate for the costs of these perturbations. Parasites represent an ecologically important stressor for animals, which may induce host plasticity. The present study examined the effects of a trematode parasite, Schistosomatium douthitti, on deer mouse (Peromyscus maniculatus) physiology, behavior and energetics. Measures were taken to assess direct pa...