Acid phosphatase activity was cytochemically detected at the ultrastructural level in the protozoan Bonamia ostreae, an intrahemocytic parasite of the flat oyster Ostrea edulis. Lead nitrate was used as capture agent for phosphate liberated during the hydrolysis of the substrate (cytidine 5' monophosphate). Lead phosphate precipitation, indicative of enzyme activity, was located in membrane-bound organelles corresponding to the cytoplasmic inclusions known as 'dense bodies'. Acid phosphatase activity level and sodium L-tartrate inhibition were measured spectrophotometrically from a purified parasite suspension. The possible involvement of this enzyme in intracellular survival of B. ostreae is discussed.