Currently recognition of molluscan diseases relies heavily upon light microscopic examination of stained histological sections of molluscan tissues Such histopathological methods are time consuming and relatively inefficient In the study of ongoing epizootics Alternatively, more effluent investigative tools are needed to accelerate disease recognition and quantification Such methods are needed to prevent control and eradicate molluscan diseases This paper reports the development of a direct sandwich enzyme-lmmunosorbent assay (ELISA) for the ascetosporan intrahemocytic parasite Bonamiao ostreae of the flat oyster Ostrea edulis. Bonamiasis is a commercially important epizootic disease of oysters A Bonamia-specific monoclonal antibody is employed in the assay The assay is performed In antibody-coated microplates employing standardized reagent and 50 µl oyster hemolymph samples This procedure was developed into a commercial product, available as a kit Based upon clinical studies, there was a 90 O/o agreement In test results when ELISAs were compared to the reference histopathologic light microscopic examination for the parasites