Pseudomonas aeruginosa is an important opportunistic pathogen, although many of its epidemiologic questions remain unanswered. In order to understand the epidemiology a valid typing method must be used to accurately distinguish strains of Pseudomonas aeruginosa . An epidemiological marker should possess three characteristics; yield the same results on repeated typing (reproducibility), be discriminating to strain differences (discrimination), and yield a small number of non-typeable strains (typability). Southern analysis using DNA probes is a technique recently developed to type the different strains which may fit these criteria. This study has been designed to compare this method with the other typing methods available. Although the results from the other laboratories are not yet available for comparison, our results have shown seventeen strains out of two hundred and twenty three isolates which exhibit undistinguishable probe reactive fragments. Other typing method's results will be compared to the DNA probes in order to determine a standard method of classification. This standard will be used to answer various questions concerning the epidemiology and pathogenicity of the organism.