SummaryLonglasting topical application of Gentamicin, in various skin lesions, induced the appearance of a Pseudomonas aeruginosa strain which was resistant to this antibiotic. Being resistant to more than 100µg/ml of Gentamicin (by disc agar diffusionand broth dilution test tube methods), this strain shows a particular lysotype. The latter may be present in the lesions together with other susceptible lysotypes. The spreading risks of such strains through the hospital wards are serious and question the topical use of major antibiotics. Resistant strans disappeared after antibiotic therapy have been discontinued.