Rickettsia-like microorganisms were described in gill endothelial cells during mass mortalities of Saint-Jacques scallops Pecten maximus in Saint-Brieuc Bay (North Brittany, France) during winter 1985-86. In order to evaluate the epizootiological significance of rickettsiosis of scallops, an analytical and descriptive epizootiological study was undertaken. The study included the geographic distribution, incidence and severity of the disease for several northern European and French scallop populations. Since the center of French scallop production is in Saint-Brieuc Bay, epizootiological investigations were concentrated in this area. Dissemination appeared to be by horizontal transmission. The pathogenic significance of the rickettsia-like organism in mortalities of P. maximus is discussed.