Three clinical isolates, K-Ps 94, K-Ps 97 and K-Ps 102, of Pseudomonas aeruginosa having R factor and showing MIC of more than 51, 200 mcg/ml to streptomycin (SM), were examined for mechanisms of SM-resistance. Among the strains, K-Ps 94 and K-Ps 102 had R factor conferring SM-resistance. In K-Ps 94, the mechanism of SM-resistance was mainly owing to SM-phosphorylating enzyme and also owing to decreased permeability by an R factor, kR94. In K-Ps 97, it was considered to be due to SM-adenylylating enzyme by the chromosomal gene but not R factor, kR97. In K-Ps 102, the reduced permeability of the cell membrane to SM by an R factor, kR102, and the reduced affinity of the ribosome to the drug by the chromosomal gene contributed to the mechanisms of SM-resistance.