Résumé : Mutants lacking NADP-linked glutamate dehydrogenase (NADP-GDH) activity have been isolated by several procedures. Complementation tests in diploids as well as tetrad analysis show that they map within a short chromosome segment, the gdhA locus, which is allelic to the ure1 locus described previously. That the gdhA locus is a structural gene for NADP-GDH is supported by two kinds of evidence. First, intracistronic complementation, as well as negative complementation were observed between some of the gdhA- mutants. This is in agreement with the multimeric structure of the NADP-GDH in Saccharomyces cerevisiae shown by Venard and Fourcade (1972). Secondly, some of the mutants at the gdhA locus have a NADP-GDH with modified properties, including: five-fold higher Km for 2-oxoglutarate, hundred-fold higher Km for NH4+, loss of inhibition by excess of substrate (2-oxoglutarate), and lower thermostability. Mutants with derepressed NAD-GDH activity have been isolated from gdhA- strains on the basis of their faster growth on ammonia as sole nitrogen source. They define the gdhCR locus, which is allelic to ure2 and usu described previously. This is a strong indication that residual growth of the gdhA- mutants on ammonia as sole nitrogen source is due to the NAD-GDH activity. © 1974 Springer-Verlag.