Two strains (O22 reference strain, 169-68, and strain 490-93 isolated from a patient with diarrhea in Thailand) of Vibrio cholerae non-O1 possessing somatic (O) antigen factors in common with V. cholerae O139 synonym ''Bengal'' are described. The O antigens of these two strains were closely related to that of V. cholerae O139 in an a,b-a,c type of relationship, but were not completely identical with serogroup O139. Therefore, both these strains are not classified into the O139 serogroup of V.: cholerae, because they have their own major antigens. As the strain 490-93 could not be placed into any of the 154 established O serogroups of V. cholerae, this strain was assigned to a new serogroup, O155. For practical use, the diagnostic antiserum prepared against the O139 reference strain (MO45, ATCC 51394) of V. cholerae must be absorbed with reference strains 169-68 and 490-93 representing serogroups O22 and O155 of V. cholerae to remove cross-reacting agglutinins of the O22 and O155 strains, respectively.