Two Shigella strains (93-119 and 95-619) isolated from stool cultures of imported diarrheal cases in Japan, did not react to any antisera of the established SHigella serovars. These strains had the typical biochemical characteristics of Shigella dysenteriae, and were biochemically identical to each other. Both strains were positive in the Serény test and other tests for invasiveness; these indicate that they can cause shigellosis in humans. The results of antigenic analysis showed that they did not belong to any of the recognized or provisional serovars, and were serologically indistinguishable. Strain 93-119 is designated as the test strain for this new serovar.