Using hybridoma technology, monoclonal antibodies (MABs) were prepared against a Rickettsiales-like organism (RLO) from the gills of the Saint-Jacques scallop Pecten maximus. The 5D7 MAB was then selected as a specific reagent suitable for an indirect immunofluorescence assay (IIFA) and evaluated for RLO diagnosis on P. maximus gill smears. Monoclonal antibody 5D7 showed no cross-reactivity with an RLO associated with a related pectinid species Chlamys opercularis.