A DEX gene from Saccharomyces diastaticus (strain BRG536 alpha DEX1) has been cloned in the hybrid vector pJDB207. The gene is included within a 3.6-kb fragment and confers production of extracellular amylo-alpha-1,4-glucosidase (AMG) and, thereby the ability to hydrolyse starch or dextrins on Dex- strains of Saccharomyces cerevisiae. The cloned gene hybridizes to three fragments produced by ClaI digestion of DNA from BRG536; one of these (11 kb) cosegregates in crosses with DEX1, while another (10 kb) is present in all Dex+ and Dex- strains examined. Accumulation of extracellular AMG by Dex+ transformants is up to five-fold that of BRG536, and escapes regulation by the CDX1 gene under conditions of excess glucose. The enzyme produced by Dex+ transformants resembles that of BRG536 with respect to Mr (approx. 150 X 10(3)) and effects of temperature and pH. The cloned DEX gene can be used as a selectable marker for introducing recombinant plasmids into wild-type strains of S. cerevisiae.