The protozoan Bonamia ostreae (Ascetospora) is the etiological agent of bonamiosis, or hemocyte disease of the European fiat oyster, Ostrea edulis (Pichot et al., 1980). Since this infection was first reported from the coast of Brittany (Comps et al., 1980), the parasite has spread rapidly along the French Atlantic coasts (Grizel & Tigé, 1982), and in the European countries including the UK, Holland, lreland and Spain (Bannister & Key, 1982; Van Banning, 1982; Polanco et al., 1984). Under the EEC Council Directive 91/67, this parasitic protozoan is included in list II of Annex A. Serious pathogen agents for molluscs that should be obligatorily declared are placed in this list