Species and antimicrobial susceptibility of nonfermenting gram-negative bacilli(NFB) isolated from clinical materials at Yonsei Medical Center during the period of September 1980 to August 1981 were analyzed and the following results were obtained. 1. NFB were isolated from 17.1% of sputum, 14.8% of pus, 5.0% of urine, 3.3% of throat and 1.4% of blood specimens. 2. Among the isolates 57.6% were Pseudomonas aeruginosa and 32.7% were Acinetobacter calcoaceticus. P. maltophilia and P. cepacia were 3.2% and 2.8% respectively. Other bacteria were rarely isolated. 3. The monthly isolation rates were high during the June to November period(8.8-12.0%), and low during the December to May period(4.2-8.4%). 4. Many of the isolates showed resistance to various antimicrobials. Although there were variations depending on the bacterial species, amikacin, colistin, gentamicin, tobramycin and co-trimoxazole showed more antibacterial activities than other antimicrobials.