Abstract: The present investigation was undertaken to determine whether differences exist in the size of cercariae, adult worms and eggs among different isolates of S. japonicum in the mainland of China. Measurements on the corresponding stages of Japanese and Philippine strains of S. japonicum were also carried out for comparison.The size of cercariae collected from pooled naturally infected snails from different isolates was shown in Table 1. The body index of Yunnan isolate, which is defined as cercaria length ×width (mm) × 1000, was found to be smaller than those of the other 4 isolates, this difference being significant at p0.01.Measurements of adult worms and eggs from different isolates were taken from various hosts infected with the same number of cercariee and for the same duration of infection. Results indicated that the mean length of mature worms and the size of tnature eggs varied not only in different host species but also among host individuals of the same species infected with the same isolate of parasite. The mean length of mature worms of S. japonicum from various hosts infected with Yunnan isolate was considerably smaller than those of the other 4 isolates (Table 2), this difference being significant at p0.05 by analysis of variance.Regarding the size (length ×width) of eggs in mice and hamsters, large size eggs were found from both Yunnan and Guangxi isolates, while in rabbits and rhesus monkeys, large size eggs were found from Sichuan parasites. Judging from the index defined as the ratio of egg width/length × 100, the eggs of the Sichuan isolate were broad and short in shape, giving higher index. Those of Guangxi and Hubei isolates were oblong, giving the lowest index (Table 3). The other two isolates from Yunnan and Anhui laid eggs between these two extremes.