Yeast mutants deficient in the constitutive ADHI (adc 1) were used for the isolation of mutants with deficiencies of the intermediary carbon metabolism, and of mutants defective in carbon catabolite derepression. Mutants were recognized by their inability to grow on YEP-glycerol and/or on ethanol synthetic complete medium. They were either defective in isocitrate lyase (ic11), succinate dehydrogenase (sdh1), or malate dehydrogenase (mdh1, mdh2), mdh-mutants could not uniformely be appointed to one of the known MDH isozymes. Homozygous mdh and sdh1 diploids are unable to sporulate. Three gene loci could be identified by mutants pleiotropically defective in many or all of the enzymes tested In ccr 1 mutants, derepression of isocitrate lyase, fructose-1,6-diphosphatase, ADHII and possibly of the cytoplasmic MDH is prevented, whereas the mitochondrial TCA-cycle enzymes, succinate dehydrogenase and malate dehydrogenase, are not significantly affected. CCR2 and CCR3 have quite similar action spectra. Both genes are obviously necessary for derepression of all enzymes tested. It could be shown that ccr1, ccr2 and ccr3 mutants are not respiratory deficient.