Thirty-four O and 19 H sera were used to test 169 strains of Plesiomonas shigelloides from several countries of three continents. The most frequent O serovar was O17 (O antigen identical with Shigella sonnei phase I) which occurred in the OH serovar combinations: O17:c, O17:d and O17:f. Some OH serovars were represented by a single strain; others, despite the small number of strains in hand, were evidently ubiquitous, having been isolated in mutually distant countries or at long time intervals. The source of our strains was most often human stools, diarrhoeal or collected at preventive examinations during epidemiological and ecological investigations, the droppings of animals (dogs, cats, pigs, sewer rats, water birds) and surface, sewer or aquarium water. The usefulness of serotyping in epidemiological investigation in the patients' environment was confirmed by a finding of two different serovars in one of our patients and her cat.