Loripes lucinalis, a small bivalve belonging to the Lucinacea superfamily living in reducing coastal sediments, possesses chemoautotrophic sulphur-oxidizing bacteria in its gill. Here, a population of putative sulphur-oxidizing bacteria is described colonizing the bivalve's inner periostracal secretion. The bacteria were particularly abundant in the vicinity of the anterior and posterior inhalant syphons. Most of the bacteria observed were oval to rod-shaped and measured 3·9±1·2 μm on average. Histology and negative transmission electron microscopy staining revealed that these bacteria contain refractile spherules similar to those seen in the endocellular gill symbionts of L. lucinalis.