Recently published reports of in-vitro, tube-dilution tests of bacterial sensitivity to gentamicin show little change since 1968 in the percentage of sensitive strains. The Bauer-Kirby procedure was useful in separation of strains of Pseudomonas aeruginosa into sensitive or resistant groups. Little correlation was found, however, between MIC and zone size, MIC and in-vivo 50% protective dose (PD50), or zone size and in-vivo PD50 within the sensitive group. The small percentage of strains resistant to 10 µg of gentamicin/ml led to a comparison between clinically resistant, clinically sensitive, and laboratory resistant strains. On the basis of growth rates, nutritional requirements, cross-resistance, virulence, and stability of resistance, two groups of resistant organisms are evident. Resistant isolates arising in the laboratory as mutations in sensitive populations under pressure of the drug have slower growth rates, revert rapidly to sensitivity, lack virulence, require specific nutrients, and are resistant to other aminoglycosides. Resistant strains found clinically are indistinguishable from sensitive ones except for an indifference to the action of gentamicin.