Arginine has long been of interest to chemists, biochemists and biologists because of its close relation-ship to urea. Srb and Horowitz (1944) isolated many arginineless mutants of Neurospora crassa and found the mutants confirmed Krebs' claim (1932) that ornithine and citrulline are precursors in the biosynthesis of arginine. With the characterization of more enzymes, there were indications that arginine biosynthesis was more complex than originally believed. When Jones, Spector and Lipman (1955) characterized ornithine transcarbamylase (OTC) as the protein catalysing the union of ornithine and carbamyl phosphate (CAP) to give citrulline, it was discovered by Fincham (1960) that no existing Neurospora mutant showed a lack of OTC, although two non-allelic mutants required citrulline. Of these, one, arg-3 was later found by Davis to lack carbamyl phosphokinese (CPK), and the other, arg-2, is still undefined. Thus an attempt was made to isolate a mutant lacking OTC, and several were obtained by Woodward (Woodward and Schwarz, 1964) by filtration and selective plating (Woodward et al., 1954) of s, the pyr-3 suppressor which was found to have a 97% reduction in OTC as compared to wild type, and also by irradiation of pyr-3 and selection of suppressed strains. The OTC-less mutants were found to be non-allelic with all known arginine mutants, to be in a single com-plementation group and thus comprise a single gene locus, to be allelic or very closely linked with s, and to be on the second linkage group of the Neurospora genome. The mutant locus has been named arg-12 and s has been renamed arg-12s. arg-12 was shown to be the structural gene for OTC. Mutants lacking OTC cannot survive, as does arg-12s, without exogenous arginine.