Sensitivity of Pseudomonas aeruginosa to cerulenin was first tested. The result indicated that this bacterium is resistant to cerulenin. Cerulenin-sensitive mutants were isolated from P. aeruginosa PML 1552 by 1-methyl-3-nitro-1-nitrosoguanidine treatment and following carbenicillin plus D-cycloserine screening. Isolated mutants were designated CSM-1 to CSM-19, and some characters of CSM-19, which showed rapid growth almost as well as parent strain in the medium without cerulenin, were examined. The cell growth of CSM-19 was greatly inhibited by 50 μg/ml of cerulenin, but when the mixture of cellular fatty acids or both cis-vaccenic acid and palmitic acid were added to the medium, the growth was partially recovered. Incorporation of radioactivity into fatty acids from [1-¹⁴C]acetate was lowered by cerulenin. Those results mean that the fatty acid synthesis of CSM-19 was decreased by cerulenin. Although cellular fatty acid composition and amount were not notably different between CSM-19 and PML 1552, CSM-19 had less phosphatidylethanolamine, and more phosphatidylglycerol and cardiolipin than PML 1552. CSM-19 was also supersensitive to several other antibiotics, especially to carbenicillin and tetracycline, when compared with PML 1552, although both strains showed identical sensitivity to D-cycloserine, polymyxin B, and chloramphenicol.