An alternative to streptomycin for treatment of possible tularemia would be useful on occasions when a patient develops a perplexing pneumonia that does not respond to initial treatment. In geographic areas where tularemia is endemic, an antimicrobial drug that is bactericidal for Francisella tularensis and is also effective against a spectrum of common pulmonary pathogens, including the Enterobacteriaceae and most strains of Pseudomonas aeruginosa, would be desirable. The purposes of this report are (1) to describe observations regarding the in vitro susceptibility of Francisella tularensis to streptomycin, kanamycin, and gentamicin; (2) to describe in vivo efficacy of these drugs in mouse tularemia; (3) to describe the results in 10 patients with tularemia treated with gentamicin. Gentamicin was bactericidal for Francisella tularensis in vitro, was effective in mouse tularemia when given in large doses, and was effective in humans when given in the standard recommended dose.