A series of 30 Saccharomyces cerevisiae aci(+) mutants (characterized as acidifying Ogur's glucose medium containing bromocresol purple) were isolated after EMS mutagenesis. All the mutants excreted acid metabolites to the medium after 24 or 48 hours of incubation. The character of the aci(+) mutations was defined using classical genetic techniques. Three of the aci(+) mutants were studied by molecular genetics techniques.