The fluorescent probe LysoTracker Red was used to examine for the presence of acidic vesicles in cercariae and schistosomula of Schistosoma mansoni. Acidic vesicles were widely distributed throughout the body of freshly transformed schistosomula and 24-h-old schistosomula but were absent from cercariae. The vesicles of freshly transformed schistosomula were undetectable after incubation with drugs that affect the functionality of acidic vesicles including monensin, ouabain, primaquine, and amiloride. In 24-h transformed schistosomula, the same effect was observed with monensin but not with ouabain, primaquine or amiloride. Praziquantel also affected the acidic vesicles of the schistosomula. We suggest that these acidic vesicles could be large lysosome-like organelles.