Trematodes belonging to the family Stomylotrematidae are intestinal parasites of birds. Despite the worldwide distribution and diversity of host species, the first intermediate host remains unknown. For a survey of parasites of Pomacea maculata, snails were collected from the municipality of Sa˜ o Vicente Fe´ rrer, state of Maranha˜ o, northeastern Brazil. In the present study, the xiphidiocercariae shed from these snails were used in the experimental infection of the water bug Belostoma plebejum. The insect mortality was observed 30 days post-infection, and the metacercariae recovered in the body cavity of B. plebejum were identified as Stomylotrema gratiosus. This is the first report of an ampullariid snail as intermediate host of stomylotrematid trematodes.