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The resistance mechanisms of more than 2,000 aminoglycoside-resistant gram-negative aerobic bacteria were estimated by a method that assigned a biochemical mechanism based on susceptibility to selected aminoglycosides. Strains from hospitals in Japan, Formosa, and Korea (the Far East) were compared with strains from Chile and the United States. Of the strains from Chile, 90% had an aminoglycoside resistance pattern indicative of the 3-N-acetyltransferase [AAC(3)-V] enzyme. Of the strains from the Far East, 78% had susceptibility patterns suggesting the presence of AAC(6') enzymes. In contrast, strains from the United States had a wider variety of resistance mechanisms including 2''-O-adenylyltidyltransferase [ANT(2'')], AAC(3), AAC(6'), and AAC(2'). Reflecting these differences in resistance patterns, the frequencies of resistance to gentamicin, tobramycin, dibekacin, and amikacin in strains from the United States were different from those in strains from the Far East. These differences seem to be correlated with different aminoglycoside usage in the two regions. In the United States, where gentamicin was the most widely used aminoglycoside, 92% of the strains were resistant to gentamicin, 81% were resistant to dibekacin, and 8.8% were resistant to amikacin. In the Far East, dibekacin and kanamycin were widely used in the past and more recently amikacin has been frequently used. Of the strains from this region, 99% were resistant to dibekacin, 85% were resistant to gentamicin, and 35% were resistant to amikacin.