A bacterial strain was isolated that possessed the biochemical characteristics of Shigella dysenteriae but did not belong to any of the established S. dysenteriae serovars, from the feces of a diarrheal case who had travelled to Turkey, Pakistan, India and Thailand for about a month. This strain was invasive to tissue culture cells and gave a positive Serény test. Preparing antiserum against this strain and examining the antigenic relationship with the established O antigens of Shigella and Escherichia coli O1-O170, we found that the strain had O antigen identical to that of the E. coli O serogroups 150. The results indicated that this strain could be a provisional serovar of S. dysenteriae. Although our strain could probably be the same serotype as that of the strains reported in Israel, this may be the first report of isolation in Japan.