Abiotic stimuli are critical in the dispersal and transmission of trematode cercariae from the first intermediate host to the next host in the life cycle. The role of 2 abiotic stimuli, light and gravity, were examined in the dispersal and transmission of the cercariae of Echinostoma caproni in a laboratory setting. Nearly 100% of cercariae placed in a vertical chamber that permitted upward migration with a marginal probability of return swam to the surface in both light and dark conditions, suggesting that a positive phototaxis is not involved in dispersal and that a negative geotaxis may be the critical component in the vertical migration. The presence of a sentinel snail (Biomphalaria glabrata) in the bottom portion of the transmission chamber resulted in a significant reduction of cercariae dispersing upward and a significantly higher intensity of metacercariae in snails under lighted conditions than in the dark. In a light/dark choice experiment the prevalence and intensity of metacercariae was significantly higher in snails in the dark. The results suggest that although a positive phototaxis is not important in the dispersal of E. caproni cercariae, a negative phototaxis enhances host finding and transmission of cercariae to snails functioning as the second intermediate host.