The O-antigens of 307 strains of mesophilic Aeromonas including 227 A. hydrophila and 80 A. caviae were studied and 44 O-serogroups defined among them. The presence of heat-labile masked antigen, which inhibits O-agglutination, was observed in some strains. As all the O-antisera prepared with these mesophilic Aeromonas strains contained some R-antibody, all diagnostic O-antisera must be absorbed with R-organisms before use. Some of the O-antigens were found to be identical or closely related to those of certain serovars of Vibrio cholerae, Vibrio fluvialis or Plesiomonas shigelloides.