Abstract: γ-glutamyltranspeptidase(γ-GTP) was found to exist in the tegument of S. japonicum, as revealed by Oaks method. Praziquantel at the concentration of 1 × 10-2 mM could inhibit in vitro the tegumental γ-GTP of male worm by 23%. By using intact worms, it was found that the activity of γ-GTP was higher than that of the tegument. The in vitro inhibitory effect of praziquantel on γ-GTP of female schistosomes was higher than that of the male worms. The authors have only measured the in vivo enzymatic activity of male worms, and the significant inhibitory effect of γ-GTP by praziquantel was found. At the same dosage, praziquantel did not show any inhibitory effect on the γ-GTP of the host kidney.