Mutants defective in carbamoylphosphate synthetase have been isolated in the fission yeasts Schizosaccharomyces pombe and Schizosaccharomyces japonicus. Their growth properties indicate a compartmentation of the carbamoylphosphate pools between arginine and pyrimidine biosyntheses, as in Neurospora crassa but unlike Saccharomyces cerevisiae. Ornithine carbamoyltransferase-minus mutants, arg3−, were also isolated in both Schizosaccharomyces species. In Schiz. pombe, a very close linkage was observed between arg3 and arg11, a gene putatively coding for acetylglutamylphosphate reductase. Arg4 and arg5, the two genes encoding the carbamoylphosphate synthetase of the arginine pathway, are also closely linked but not adjacent.