The fine and ultrafine structure of the capsules surrounding the acanthocephalan Corynosoma strumosum in natural paratenic hosts of three species--the whitespotted greenling Hexagrammos stelleri, Steller's sculpin Myoxocephalus stelleri, and the saffron cod Eleginus gracilis--was studied. The results of this study, together with earlier data, support the hypothesis that the structure of the capsule depends on the paratenic host species. Three types of capsules differing in the ratio of fibroblasts and inflammatory cells have been identified: fibroblastic (in Japanese smelt, rainbow smelt, and saffron cod), intermediate (in whitespotted greenling), and leukocytic (in Steller's sculpin and the yellowfin sole). It was assumed that the structure of the capsule is determined by the degree of mutual adaptation of organisms in a given host-parasite system.