An infection by a yeast in a colony of American cockroaches, Periplaneta americana (L.), causes extreme paralysis that leads to death. Examination of hemolymph revealed the presence of ellipsoidal yeast cells that have been identified as a new Candida species. The pathogen has been given an accession number and made a part of the permanent USDA collection of yeasts. Electron microscopy of cockroach hemolymph containing the Candida cells indicated that of the two main cockroach hemocyte types, granulocytes are more susceptible to damage than plasmatocytes, and cytoplasmic disintegration proceeds much faster than that of the nucleus. Cockroach hemocytes respond to presence of the yeast by both phagocytosis and encapsulation. Consideration of the possibility of using the fungus in biological control of the cockroach is premature at this time because sibling species of Candida are human pathogens. Extensive toxicity and allergenicity tests are needed. Because not all cockroaches succumb to the Candida infection, further study of parasitic interactions are valuable in elucidating the nature of immunological responses in the American cockroach.