The fine structure of the tegument of three trematode species, Schistosoma mansoni, Dicrocoelium dendriticum, and Fasciola hepatica, was studied by means of light scanning (SEM) and transmission electron microscopy (TEM) after in vitro exposure to 0, 1, 10, and 100 micrograms/ml of the anthelmintic praziquantel for 5, 15, 30, and 60 min. In S. mansoni and D. dendriticum the resulting vacuolization of the tegument was confined to numerous small areas scattered all over the surface of the parasites and this finally led to the disruption of the apical tegumental layer. No changes were found in the tegument of F. hepatica after treatment with praziquantel.