During a study of Rickettsiales-like organisms (RLO) in Pecten maximus gill, histological observation of free RLO in hemoclymph led us to consider interactions between the pathogen and the hemocytes. Firstly, the production of oxygen radicals by hemocytes was established using a luminol-enhanced chemiluminescence method with zymosan axs stimulant. The effects of serveral inhibitors of oxidative metabolism were investigated. The internalization of RLO into hemocytes was demonstrated by electron microscopy but was not related to chemiluminescent activity. The possible involvement of RLO acid phosphatase as the inhibitor of respiratory burst was assessed.