The measurements of the eggs of S. japonicum from dogs, cats, hamsters and albino mice infected with the Chinese, Formosan, Japanese and Philippine strains were compared. The size and shape of the eggs showed significant differences even between host-individuals of the same species infected with the same strain of the parasite. Between strains, if the mean of the measurements of a number of eggs from a host-individual was obtained and the collective means from a number of host-individuals infected with one strain were compared with those of the other strains, significant interstrain differences were found in the length, width and especially in the index of the eggs. The confidence intervals of the mean of the means of the index of the eggs were very distinct, i.e., there was no overlapping among the four strains. The variations of the eggs between host-species were significant in certain cases. Accordingly the interstrain comparison of the size and shape of the eggs of this parasite should be made for the eggs from the same host-species.