The ultrastructure of the pathogen producing yellow-spot disease in the dendronotacean nudibranch Tritonia diomedea is described. A lamellar wall of thin scales, the morphology of the Golgi body, the presence of a putative bothrosome, and production of zoospores with mastigonemes indicate that the parasite is in the Thraustochytriaceae, a family of marine protists often included in the lower fungi. Host amoebocytes became greatly flattened and formed a lamellated wall around the parasitic cells, which finally were enclosed in a dense, thick-walled, acellular capsule where they were generally seen to be necrotic. The extensive encapsulation and necrosis suggested that the gastropod may be an unnatural host. The large dendronotacean nudibranch Tritonia diomedea is used extensively in neurophysiological studies. For a number of years, many specimens delivered to the Friday Harbor Laboratories on San Juan Island, Washington, have been afflicted with a parasitic infection known locally as the yellowspot disease or ringworm. The condition is characterized by the occurrence in the subepidermal tissues of yellow spots as large as 1.50 mm in diameter, and occasionally by erosion of surface layers. We describe here by electronmicroscopy the parasite and its encapsulation by amoebocytes of the host. Our primary purpose was to identify the pathogenic organism. Ultrastructural evidence indicates that the pathogen is a member of the Thraustochytriaceae, which are marine protists often included within the lower fungi. A secondary objective was to obtain information on the functioning of the encapsulating amoebocytes. Little is known about encapsulation of unicellular parasites in gastropods. Two relatively recent studies (Harris, 1976; Sullivan et al., 1978) show that metazoan parasites that remain viable when encapsulated in various snails are surrounded by amoebocytes that retain their cellular identity. In the present study, the amoebocytes surrounded the parasites as is expected in gasReceived 5 May 1981; revised 21 July 1981; accepted 19 October 1981. * Present address: Botany Department, The University of Georgia, Athens, Georgia 30602. tropods; but in older capsules they became dense and disappeared, and the parasites often were seen to be necrotic and encased by a thick, acellular wall. There are few reports on protistan or fungal endoparasites, or possible endoparasites, in the opisthobranchiate gastropods. The majority of these reports are notes within studies on other topics. Baba (1937) and Millott (1937) observed ciliates in the tissues of two nudibranchs. Eakin et al. (1967) reported small cells presumed to be symbionts in the eye of the nudibranch Hermissenda crassicornis. McLean (1978) described similar cells in many tissues of the sacoglossan Alderia modesta. Bruel (1940) detected an unidentified sporozoan infection in the sacoglossan Placida sp. A flagellate that parasitizes egg masses of doridacean nudibranchs was described by Zerling (1933), and Eliot (1910) briefly noted that vegetable growths described as Phycomycetes occasionally invade nudibranchs. The present study, therefore, is a contribution to a field in which little information is avail-