Psorospemum haeckeli, a common parasite of the European noble crayfish Astacus astacus, occurs as an egg-shaped spore of ca 100 X 60 X 60 pm, particularly within the collagenous layer of the thoracic arteries and the surrounding subepidermal connective tissue. Smaller numbers are also found within the connective tissues of virtually all organs. The morphology and histology of these spores was investigated by microscopic techniques and the composition of their major structural components was identified by histochemistry. Each spore is enveloped by a layer of connective tissue which is assumed to derive from the host. It varies in thickness and composition among the infected organs. The spore proper consists of a 3-layered shell enclosing many globules. The outer shell layer is composed of solid, collagen-and elastin-negative, proteinaceous plates separated by sutures. The medial shell layer appears less compact and includes glycoprotelns or neutral mucopolysaccharides. The inner layer is composed of 2 membranous structures including glycoproteins, glycolipids, and collagen-and elastin-like proteins. The interior of the spore contains lipid globules of variable size, intensely staining nonlipid globules with crystalline and vacuolar inclusions, and 2 closely associated nuclei. In most of the crayfish investigated some of the spores were encapsulated by haemocytes and melanized.