Summary Three strains of similar antigenic and cultural behavior were isolated from cases of diarrhoeic infection. Their cultural properties suggest their inclusion in the genus Shigella. They have a characteristic major antigen different from any of the presently known Shigellæ; their secondary antigens are essentially similar to those found in phase B of Sh. paradysenteriæ type IV. The designation Shigella rio is suggested for these microorganisms.