Unsaturated fatty acid mutants of Neurospora fall into two loci located on linkage group V. The growth requirements of these strains can be satisfied by any of the naturally occurring fatty acids of the wild-type organism. Compositional analysis of the delta fa mutants supplemented with C18 mono-, di- and tri-unsaturated fatty acids suggested that the delta fa mutations affect the delta 9-desaturase. This possibility will be explored by comparing the levels of this activity in extracts of wild type and the delta fa mutants. The delta fa-2 locus is composed of seven complementation groups suggesting that multiple peptides may be responsible for the delta 9-desaturase activity. Attempts will be made to obtain in vitro complementation of the delta 9-desaturase by mixing of detergent-treated extracts from mutants of different complementation groups. The delta 12-desaturase will also be characterized. This latter activity is obscured by the dominant delta 9-desaturase in wild-type extracts. The role of fatty acid compositional changes on both activities in strains supplemented with the appropriate unsaturated fatty acids will be assessed. In this manner, the delta fa mutants can greatly facilitate the elucidation of the various steps of fatty acid desaturation and the regulatory controls of this reaction sequence.