Serotyping of Pseudomonas aeruginosa offers a specific, reproducible means of subspeciating this genus. It can be utilized for investigation of various parameters, such as prevalence of specific serotypes, predilection of these serotypes for sites in the patient, the relationships of environmental strains to colonized patients, and the prevalence of specific serotypes over time. It also allows comparison of antibiograms with specific serotypes. In combination with bacteriophage or bac- teriocin typing, serotyping offers the finest means of differentiating epidemic strains. A number of investigators have developed sche- mata for serotyping of Pseudomonas aeruginosa to enable the differentiation of strains within the