Murine monoclonal and polyclonal antibodies to antigens of Bonamia ostreae were assessed for serological comparison of parasites associated with different geographical origins of flat oysters, Ostrea edulis, and with Tiostrea lutaria. All Bonamia isolates from O. edulis reacted similary with polyclonal and monoclonal antibodies. The parasite associated with Tiostrea lutaria which lacks reactivity with 20 B2 MAB must be considered as a serologically distinct form of B. ostreae. © 1988 IFREMER-Gauthier-Villars.