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Frontonia sp. was repeatedly found in sea water samples collected from a shore near Leghorn (Ligurian sea). Endosymbiotic rod shaped bacteria 5-6 μm long were found in the cytoplasm of each Frontonia sp. specimen examined at the fluorescent microscope following DAPI staining procedure. These bacteria, as revealed by electron microscopy, are contained in vacuoles (one or two symbionts are present in each vacuole) and are covered by numerous flagella which are in close contact with the vacuolar membrane. Flagellated symbiotic bacteria in ciliates have been seldom described. All the cases referred in the literature concern Paramecium, a genus that mainly consists of fresh-water species. This is the first record of flagellated bacteria living in a marine ciliate species belonging to a different taxon. The fact that these symbionts have been constantly observed in just collected Frontonia and are maintained in specimens grown in the lab for at least one month, suggests that we are not dealing with an occasional relationship. The abundance of cytoplasmic vesicles and glycogen particles, around and sometimes within the vacuoles, suggests a movement of metabolites between the host cytoplasm and the symbionts. The bacteria maintained their motility once outside the host cell.