[URE3], a non-mitochondrial non-mendelian mutation which modifies drastically yeast nitrogen metabolism has been genetically studied. Cytoduction experiments show definitely that the inheritance of the determinant is not linked to the nucleus. The maintenance of the [URE3] determinant seems controlled by the product of a conventional nuclear gene (ure2) which is itself involved in nitrogen metabolism. The (ure2) mutation alone gives the same phenotype as [URE3] but it is impossible to obtain a stable recombinant containing simultaneously the (ure2) mutation and the [URE3] determinant. Application of the Newcombe respreading experiment demonstrates that the [URE3] mutational event occurs before the selection procedure and is therefore not strictly adaptative. Nevertheless, the nature of the selection medium changes considerably the frequency of the [URE3] mutants recovered.