The intracellular level of .beta.-hydroxydecanoyl thio ester dehydrase, the product of the fabA gene of E. coli, was increased by isolation of a putative promotor mutant (termed fabAup) or by molecular cloning of the wild-type fabA gene into plasmid pBR322. The fabAup and plasmid-carrying strains overproduced dehydrase by .apprx. 15- and 10-fold, respectively. The phospholipids of all strains that overproduced the dehydrase contained significantly higher levels of saturated fatty acids than isogenic strains producing a normal level of dehydrase. No increased levels of unsaturated fatty acids were observed. Although the dehydrase is required for unsaturated fatty acid synthesis, the level of dehydrase activity in wild-type cell does not limit the rate of unsaturated fatty acid synthesis. The introduction of a plasmid carrying the structural gene for .beta.-ketoacyl acyl carrier protein synthase I into a fabAup strain overcame the effect of dehydrase overproduction on fatty acid composition.