The in vivo effects of a single dose of levo-praziquantel, 75 mg/kg in PEG 400, on the tegumental surface of adult S. japonicum were compared with the effects of a single dose (150 mg/kg) of the mixed isomer preparation, using scanning and transmission electron microscope. Worms were recovered from mice at 10 min, 30 min, 1 hr, 4 hr, 12 hr, 24 hr and 48 hr after treatment. After 10 min exposure to either compound, the tegumental folds and sensory organelles were swollen and the tegument vacuolated. After 12-24 hr, the surface was eroded and exfoliated with exposure of intrategumental and/or subtegumental tissues and attachment of leukocytes to the denuded areas. Vehicle controls were normal throughout the time period examined. These studies demonstrate that the levo isomer of praziquantel causes acute structural damage to the tegument similar to that seen with the mixed isomer preparation.