Abstract: 　Objective To examine the antagonistic interactions between Schistosoma japonicum and Exorchis ovariolobularis in the snail host, Oncomelania hupensis . Methods The infection rate and average number of cercariae of S japonicum were examined in O hupensis which had been superinfected with S japonicum and E ovariolobularis .The experimental snails were histologically observed by paraffin section. Results When snails were fed with eggs of E ovariolobularis 37 days later than S japonicum infection, the infection rate of S japonicum (52 9%) was significantly lower than that of the single S japonicum infected control (75 9%). When E ovariolobularis infected snails were exposed to S japonicum miracidia at different intervals, the infection rates of S japonicum were 64%, 66 7%, 65 2%, 56 4% and 57 1%, respectively. On the other hand, the infection rate of S japonicum in the single infected control was 90 5%. There existed significant differences. Furthermore, the average number of S japonicum cercariae shed from superinfected snails were significantly less than that of single S japonicum infected control. The histological observation showed that the larvae of S japonicum in superinfected snails were sluggish in development, irregular, shrunken, deformed, relatively small in number as compared with the control. The sporocysts contained few germ balls or immature cercariae and most of the digestive glands of superinfected snails were atrophied. Conclusion There is antagonism between E ovariolobularis and S japonicum in O hupensis , and E ovariolobularis is dominant in this situation.

Key words: Schistosoma japonicum, Exorchis ovariolobularis, antagonism, Oncomelania hupensis