Larvae of Schistosoma japonicum could be blocked in Oncomelania hupensis with pre-infection by larval Exorchis trematodes (Tang et al., 2008, 2009). The present manuscript reports the result of comparison between the two different developments of larval S.japonicum in O.hupensis snails with and without pre-infection by larval Exorchis trematodes. Of 300 (invader rate 26.74%) abnormal larvae of S.japonicum for 4-82 days old were found from 28 (73.7%) O.hupensis snails which were pre-infected with Exorchis eggs, their body structure were unusual and developed astray at early mother sporocyst stage. From 25 (69.4%) O.hupensis snails without pre- infection by larval Exorchis, 67 (invader rate 13.96%) normal mother sporocysts of S.japonicum for 5-61 days old in different development stages and many early or middle stage daughter sporocysts were found, in the 75th day after infection many mature daughter sporocysts and cercariae of S.japonicum were found. Different increase of lymphocyte in body tissues of snails were found from the two experimental O.hupensis snail groups.