Sequential development and histopathologic effects of Fasciolopsis buski larvae were examined in two species of pulmonate snail, Hippeutis umbilicalis and Segmentina trochoideus. Miracidial attachment and penetration began 5 min post-exposure (PE). On day 2 PE, the sporocysts rapidly increased in size and over days 3-7 PE, they transformed into mother rediae which migrated to the ovotestis on day 10 PE. In both species of snail, daughter rediae were initially observed on day 17 PE, the first cercariae emerged on day 21 PE, and from that day onward daughter rediae were the dominant larval form occupying the ovotestis. Histopathologic changes were confined exclusively to the ovotestis and included lytic lesions due to mechanical damage, and disappearance of all cellular elements. No inflammatory responses were observed in the ovotestis, and no degenerated larvae of any kind were observed in the snails. Infected snails did not produce egg masses and lived no longer than 35 days. The lack of differences in miracidial recognition and penetration, and in the temporal, sequential, and spatial development of F. buski larvae in H. umbilicalis and S. trochoideus indicate that both these snail species are equally susceptible to F. buski.