An integrated GAL1-lacZ fusion provided a useful phenotypic marker for the gal80- regulatory mutation in Saccharomyces cerevisiae. On minimal glucose plates containing a beta-galactosidase indicator, a GAL80 strain containing the fusion gave white colonies, whereas a gal80- strain gave blue colonies. This color difference was used to isolate the GAL80 gene from a plasmid bank by complementation of the gal80- mutant. The putative GAL80 gene was located on a 2.6-kb HindIII-SalI fragment and has been subcloned into an integrating vector. Genetic analysis showed that the clone integrated at the GAL80 locus. A deletion that covered the entire GAL80 region was constructed in vitro and transplaced into the yeast genome to give an isogenic pair of GAL80 and gal80 deletion strains. Glucose repression of a GAL1-lacZ fusion was normal in the gal80 deletion strain, implying that the GAL80 gene product is not involved in glucose repression.