Circulation of different antigenic variants of P. aeruginosa in a surgical hospital was studied. In this study the process leading to the formation of pathogenic hospital strains, determined by time and location, from some serovars is demonstrated. The study also established that the department of the hospital where the selection of hospital strains mainly occurred was the resuscitation ward. Some pyoseptic infections of P. aeruginosa etiology with fetal outcome were found to be caused in most cases by hospital strains characteristic of the hospital in the period under study.