Abstract: In order to understand the immunity induced by NTG attenuated cercariae of S. japonicum, investigations were carried out on the survival rate of mice, worm reduction rate, liver and spleen granulation etc. at different intervals between immunization and challenge (4, 6, 8, 10 and 12 wk) in mice exposed percutaneously to NTG attenuated cercariae (30 μg/ml NTG for 15 min). Scanning electron microscopy on tegumental surface of schistosomes from immunized mice was also performed. Results showed that better protection was achieved at a challenge time of 8 wk after initial immunizing infection as demonstrated by the higher worm reduction rate and the longer survival rate with a lower degree of liver granulation. It is suggested that acquired immunity did develop against a challenge in mice exposed to NTG-attenuated cercariae, but there were different levels of immunity at different times post-immuniza-tion. Thus week 8 appeared to be the optimal time for acquired immunity developed against a challenge infection. The posibility of induction of different levels of immunity was discussed.