The ultrastructure of the alga and fungus of the lichen Dermatocarpon hepaticurn and their relationship has been studied. The contact between the symbionts was found to be of a different type from that found in other lichen species, and to some extent comparable with the relation of obligate parasites and their host. The anatomy of the thallus and the distribution pattern of the components in the thallus has been revealed by examining serial sections.