Three new major somatic antigens of Pseudomonas aeruginosa have been described and added to the 17 existing groups in the International Antigenic Typing System (P. V. Liu, H. Matsumoto, H. Kusama, and T. Bergan, Int. J. Syst. Bacteriol. 33:256-264, 1983). These newly recognized antigens are present in some strains of P. aeruginosa that have been used as type strains of various typing schemata but have not been recognized because of cross-reactions of these strains with antisera of other serogroups with minor antigens. The lists of corresponding serogroups of various serotyping schemata have been revised accordingly.