The objectives of this study were to describe the effect of light wavelength and osmolality on the vertical swimming distance of the cercaria of Proterometra macrostoma. Significant differences were found in the average swimming distances and electrical burst activity in the tail of cercariae exposed to red vs. blue, green or white light, but no difference was observed when the last three colors were compared to one another with one exception. Cercarial swimming decreased slightly after 12 hr PE (post-exposure) in artificial pond water (15 mOsm) and artificial snail water (102 mOsm). By contrast, a significant reduction in swimming distance was observed in distilled water (0 mOsm) and artificial snail water with two different concentrations of mannitol (180 and 267 mOsm).