Fatty acid elongation defective mutants were isolated from Saccharomyces cerevisiae by mutagenizing strains that were defective in fatty acid synthase (FAS) activity. Cells of the fatty acid synthase-defective strains can grow when supplemented with tetradecanoic acid (14:0) due to the presence of membrane bound elongation systems that can extend the 14 carbon fatty acid to longer chain species. After mutagenesis and rescue on medium containing a mixture of 14:0, 16:0 and 18:0, cells were screened for their inability to grow on medium containing only 14:0. From 150,000 colonies, four stable isolates were identified, all of which appear to represent the same complementation group. Gas chromatography of lipid extracts from mutant elo1-1 (designated as elongation defective) cells grown with long or medium chain fatty acids indicates that it fails to efficiently elongate (12, 13, or 14) carbon fatty acids. A gene disrupted fas2Delta::LEU2;elo1Delta::HIS3 mutant incorporates 14-18-carbon fatty acids into membrane lipids, indicating that fatty acid transport is not affected by the mutation. Molecular cloning and sequence analysis of the ELO1 gene suggests that the encoded protein is a membrane bound polypeptide that contains at least five potential membrane spanning regions and a presumptive NADPH binding site. Analysis of the ELO1 mRNA levels indicates that the gene is expressed in cells grown on fatty acid deficient medium. It is rapidly induced in wild type cells that are supplemented with 14:0 and is repressed when cells are supplied with 16- and 18-carbon fatty acids.