In vitro interactions between intramolluscan stages (sporocyst, daughter rediae, and metacercariae) of the trematode parasite Echinostoma paraensei and adherent hemocytes from the gastropods Biomphalaria glabrata (intermediate host) and Helix aspersa (non-host) were visualized by time-lapse videomicroscopy. Hemocytes of either species not exposed to E. paraensei displayed extensive mobility and activity of cellular extensions. Image analysis disclosed no significant change in the total surface area occupied by hemocytes in a selected field of view over 2 hr. Echinostoma paraensei exerted life stage-specific effects on the behavior of B. glabrata hemocytes; the cells moved away from sporocysts and daughter rediae but not encysted metacercariae. In the presence of sporocysts, hemocytes rounded up, whereas hemocytes adjacent to rediae assumed a stringy, beady appearance. Hemocytes close to the parasite were affected more rapidly than more distant cells. In 2 hr, a hemocyte-free "halo" formed around the parasite larvae, significantly reducing the hemocyte-occupied surface area (to 43% by sporocysts and to 70% by rediae). The changes induced by sporocysts and rediae are similar to those noted in both in vivo and in vitro studies of the B. glabrata-E. paraensei model system and are interpreted as manifestations of parasite-mediated interference with host hemocyte function. Helix aspersa (non-host) hemocytes were not affected, suggesting that E. paraensei-mediated effects on hemocytes exhibit a degree of specificity.