A total of 166 strains of Gram-negative bacilli comprising 65 Klebsiella species, 22 Escherichia coli, 39 Pseudomonas aeruginosa, 20 Proteus species, 18 other Pseudomonas species and 2 Salmonella typhi, were isolated from clinical specimens in the University College Hospital, Ibadan. Antimicrobial susceptibility testing and plasmid profiling of the strains were done. Ceftazidine had the highest antibacterial activity of 83.1% compared to the fluoroquinolones followed by ciprofloxacin (78.3%), pefloxacin (69.9%) and ofloxacin (56%) respectively. Of the 166 strains, 44 were found to be resistant to most of the antimicrobial agents tested. Resistance to ofloxacin was common among the resistant strains. The resistant strains harboured plasmids with molecular sizes ranging from 6.6kb to 17.4kb and were grouped into five plasmid profile groups. Transformation experiment showed that 59.2% of the resistant strains carried a common R-plasmid of size 10.7kb. Resistance to ciprofloxacin and pefloxacin were found to be plasmid borne.