Significant mortalities have been observed in populations of Saint-Jacques scallops Pecten maximus, in association with numerous intrace1lular colonies of Rickettsiales-like organisms (RLO) in the gi1ls. Isolation of the procaryotes from infected tissues and their subsequent purification were achieved by a specific protocol based on differential and isopycnic centrifugations on sucrose and renografin density gradients. The purity and integrity of purified procaryotes were verified by electron microscopy. Specific polyclonal antibodies were prepared for serological characterization. Fifteen major constituent proteins were electrophoretically identified, their molecular weights ranging between 148 and 16 kD. Ten enzyme activities were biochemica1ly identified, including acid phosphatase and catalase, which could be related to procaryote pathogenicity.