An account is given of the distribution of 958 strains of Ps. aeruginosa among 14 O groups, investigated by means of a tube agglutination technique and heat-treated antigen suspensions. Use has been made of unabsorbed O sera corresponding to Habs 12 O groups, an O group 13 from Sandvik's material, and an O group 14 prepared with a strain from the present material. 75 per cent of the strains, which originated chiefly from patients in a hospital environment, could be O group determined. 55 per cent of the strains belonged to O groups 6, 2, 5, 3 and 11. Strains belonging to O groups 2, 5 and 6 were the ones most frequently occurring in a hospital environment. The results showed that strains belonging to O groups 2 and 5 were more frequent than O group 6 strains in a hospital environment and that they colonized more easily than O group 6 strains wounds, urinary tract, ears, pleura and lower respiratory tract, whereas strains belonging to O groups 2 and 5 and O group 6 were equally frequent in the upper respiratory tract and in the large intestine (stools). The frequent occurrence of strains belonging to O groups 2 and 5 in samples from blood and central nervous system can be explained by the finding of these O group strains at sites from where Ps. aeruginosa is presumed to advance into the circulation and central nervous system.