SYNOPSIS. Different species of larval trematodes that utilize the same intermediate host characteristically exhibit distinctive patterns of response to environmental fluctuations. Not only are the responses distinctive for each species, but also they cannot be correlated with that of the host. The physiological responses of the molluscan host to physical changes in the environment, however, are modified significantly by the presence of a larval trematode infection. The thermal metabolic acclimation patterns of infected Nassarius obsoleta are quite distinct from those of non-infected N. obsoleta. Thex activity patterns during thermal acclimation of cytochrome c oxidase from digestive glands of snails infected with different species of larval trematodes indicate that each species of larval trematodes differentially influences the hosttissue, so that the cytochrome oxidase system does not respond to temperature in the same way as does uninfected host tissue.