The investigation concerns biologic, morphologic, and taxonomic characteristics of cestodes (tapeworms) of the genus Echinococcus, of which three species in the larval stage cause hydatid disease in man and, to a limited extent, of the closely related genus Taenia. Studies on the larval stage of Echinococcus spp., which is non-infectious under laboratory conditions, involve investigations that would: 1) define differential morphologic characteristics of diagnostic value; 2) elucidate the pattern of asexual proliferation in the recently discovered species E. vogeli, and thereby establish the mechanism of pathogenesis in man; 3) apply cytogenetic methods for taxonomic purposes; 4) establish the differential characteristics and taxonomic status of the two major biologic strains of E. granulosus, the cause of cystic hydatid disease; 5) define effects of long-term chemotherapy with mebendazole on tissues of the larval E. multilocularis, the cause of alveolar hydatid disease in man; and 6) define further opportunities for investigation. The objective of this work is to obtain new information that would contribute to better means for the diagnosis, prevention, and control of this complex of parasitic diseases.