Cvc- mutants of Escherichia coli are deficient in the synthesis of cis-vaccenic acid and in the temperature control of fatty acid synthesis. In this communication, it is demonstrated that these mutants lack beta-ketoacyl-acyl carrier protein synthase II. The deficiencies in cis-vaccenate synthesis and synthase II are shown to be due to a lesion in the same gene, fabF. Lesions in the fabF gene are found to affect growth only when the strain also carries a lesion in the fabB gene, the structural gene for beta-ketoacyl-acyl carrier protein synthase I.