The complete nucleotide sequence of the ARG3 structural gene encoding the monomer of the trimeric ornithine carbamoyltransferase (OTCase) (EC 2.1.3.3) has been determined. It consists of 338 codons with a corresponding molecular mass of 37842 Da. Comparing OTCases from Escherichia coli, yeast, Aspergillus, rat and man emphasizes peculiarities of the yeast enzyme but also brings to light an important degree of conservation between these proteins. Comparing the various OTCases with E. coli aspartate carbamoyltransferase (ATCase) (EC 2.1.3.2) confirms the evolutionary relationship previously noted between the two types of carbamoyltransferases and points to residues probably involved in catalysis and structural folding in OTCases.