113 pyrimidine auxotrophs, unable to synthesise UMP have been selected in Aspergillus nidulans. These mutants can be classified by complementation into eight groups, and genetic analysis has shown that five loci are involved. One complex locus consists of the mutually complementing pyrA, pyrB and pyrC groups, as well as the cis-dominant pyrN group, members of which do not complement with members of the A, B or C groups. pyrA mutants have been shown to lack CPSase-ur, pyrB and pyrC mutants have been shown to lack ACTase, and pyrN to lack both these enzymes. This locus appears to code for products which form an enzyme aggregate. The four simple loci, as well as the complex loci have been located genetically, and distinguished from one another on the basis of accumulation of pyrimidine precursors in vivo. The synthesis of ACTase has been shown to subject to end-product repression.