Five Shigella strains isolated from stool cultures of five sporadic imported diarrheal cases in Japan during 1999-2001, did not react to any antisera of the established Shigella serovars. These strains had the typical biochemical characteristics of Shigella boydii, and were biochemically identical. All strains were positive in a PCR assay and a cultured-cell invasion test for invasiveness; these indicate that they can cause shigellosis in humans. The results of antigenic analysis revealed that they did not belong to any of the recognized or provisional serovars, and were serologically indistinguishable. Strain SM00-27 is designated as the test strain for this new S. boydii serovar.