In Saccharomyces, the enzymes used to convert galactose to glucose are specified by three coordinately expressed, tightly linked genes, GAL7, GAL10, and GAL1. These genes are induced by galactose and are controlled by the positive regulator gene gal4 and the negative regulator gene gal80. GAL81 mutations, which are known to alter the gal4 protein, produce a constitutive phenotype. We have cloned fragments of Saccharomyces carlsbergensis DNA that span 26.3 kilobases surrounding the three clustered GAL genes. About 5 kilobases of the sequence was determined, which includes the entire GAL1 gene, the two intercistronic regions, and portions of the coding sequences of GAL10 and GAL7. Some amino acid homology between the GAL1 gene product, galactokinase, and the Escherichia coli galactokinase was detected. By using various Saccharomyces DNA fragments, the accumulation of GAL1 and GAL10 RNA in yeast cells after induction with galactose was studied. Our results, using wild-type, gal4-, gal80-, and GAL81-1- yeast cells, support the hypothesis that control is exerted at the transcriptional level.