Plagiorchis elegans has been shown to decrease the fecundity and survivorship of the incompatible snail host Biomphalaria glabrata. Furthermore, a prior infection with P. elegans was shown to render the snails resistant to the compatible parasite Schistosoma mansoni. Here, we test the hypothesis that infection with P. elegans stimulates the immune system of B. glabrata. Our findings indicate that infection by P. elegans significantly increased the number of free hemocytes in the hemolymph of B. glabrata by an average of ∼60%. Furthermore, this immuno-stimulated state lasted from the first day post-infection (PI) to some time between 7 and 21 days PI. This is one of the few reported examples of a parasite stimulating the immune response of an incompatible host.