Female mice of nine inbred strains (A, AKR, BALB/c, C3H, C57BL/6, C57BL/10, CBA, DBA/1 and DBA/2) and H-2 congenic B10.D2, at 9–10 weeks of age, were infected with larval Echinococcus multilocularis by trans-portal injection of hydatid homogenate. Parasitized livers were histologically examined 9 or 13 weeks after infection. Hydatid development was quite different among mouse strains. Multivesiculation was prominent in C57BL/10, DBA/1, C57BL/6 and BALB/c. Protoscoleces were well developed in DBA/2, AKR, DBA/1 and CBA. H-2 congenic B10.D2, which has the background genes of C57BL/10 except for the H-2d gene of DBA/2, resembled C57BL/10 in prohibiting the development of protoscoleces. These data suggest that the qualitative difference in hydatid development may be regulated by non-H-2 gene(s).