Heated saline extracts of 89 strains, and (1) supernates of phenol-water extracts (L1 fractions), (2) purified lipopolysaccharide, (3) trichloracetic-acid (TCA) extracts, and (4) sodium-hydroxide extracts of 23 strains representing all Pseudomonas aeruginosa O antigens were subjected electrophoresis. Precipitation lines obtained with homologous and heterologous antisera were evaluated by electrodensitometric measurement. The characteristics of the immunoelectrophoretic groups established were as follows. Group I: two lines running at different rates towards the anode; three subgroups on the basis of the behaviour of alkali-treated antigens. Group II: triple line at the starting well, alkali sensitive. Group III: triple line at the starting well, alkali resistant; two subgroups according to reactivity or non-reactivity of L1 fractions. Group IV: triple line on the cathode side, alkali resistant, L1 fraction non-reactive. Group V: single line on the anode side, alkali sensitive, L1 fraction and TCA extract non-reactive. O antigens identified by agglutination corresponded closely with the immunoelectrophoretic pattern: strains with identical O antigens or sharing major somatic components fell, with one exception, into the same immunoelectrophoretic group.