The in vitro activity of the aminoglycosides tobramycin and gentamicin was determined through a study using 100 clinical Pseudomonas aeruginosa and 3 international standard strains. Tobramycin was clearly more active against these clinical strains. Comparing the present results with those obtained 3 years ago using gentamicin against 50 strains of the same baterial group, we noticed an evident loss in activity. We report the finding of a gentamicin-resistant tobramycin-sensitive P. aeruginosa strain; this resistance might be due to a chromosomic change rather than to an R-factor.