Plasmid conjugation experiments of 34 gentamicin-resistant strains of K. Pneumoniae were done. The resistant genes of 31 strains (91%) were transferred partially or completely with a frequency of 10(-4)-10(-6). The results of plasmid analysis in 63 strains of K. Pneumoniae showed that 85kb and 58kb plasmids carrying resistant genes were responsible for bacterial resistance. 13kb plasmids or the smaller ones were not related to bacterial resistance. In spite of the aminoglycoside modifying enzyme, The profiles were different, but the plasmid patterns were almost the same.