Infectivity studies were done on laboratory-reared Biomphalaria glabrata juvenile (neonatal) snails exposed to cercariae of Echinostoma trivolvis. Of the 40 snails, 0.7-1.0 mm in shell diameter, which were exposed to a total of 1,000 cercariae for 24 hr, all were infected with 1-29 viable encysted metacercariae per snail. Biomphalaria glabrata, 1-2 mm in shell diameter, were used to determine the effects of an infectious dose of 25 cercariae/juvenile on subsequent snail survival. The median survival time of 24 exposed snails occurred between day 3 and 4 postinfection, at which time more than 90% of 24 control (unexposed) snails were alive. The high mortality in the E. trivolvis-infected B. glabrata juveniles was probably related to cercarial penetration and encystment in the snail tissue, although histological examination of snails after exposure would be required to determine the actual cause of the observed mortality.