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Alveolar and cystic hydatidosis are caused by infection with the larval stages of Echinococcus multilocularis and Echinococcus granulosus, respectively. A host-protective antigen has been identified in E. granulosus. Here we identify the presence of a closely related protein in E. multilocularis, characterize and express a cDNA encoding the antigen (designated EM95), determine the structure of the em95 gene, and demonstrate that the EM95 recombinant protein can be used to induce significant levels of protection against challenge infection with E. multilocularis eggs in mice. Echinococcus multilocularis is a cestode parasite that causes the often-fatal disease alveolar echinococcosis in humans (23). The normal life cycle of the parasite occurs between foxes, dogs or cats (the definitive hosts), and various species of rodent that act as natural intermediate hosts. Humans become in-fected by accidental ingestion of the parasite’s eggs from the feces of an infected definitive host. Echinococcus granulosu