Maltose fermentation in Saccharomyces species requires the presence of at least one of five unlinked MAL loci: MAL1, MAL2, MAL3, MAL4 and MAL6. Each MAL locus is complex consisting of at least three genes: a trans-acting activator, a maltose permease, and maltase. All the MAL loci show homology to each other both at the sequence level as determined by Southern transfer analysis and at the functional level as determined by complementation. We describe the organization of the MAL loci in yeast and the basic features of their regulation. The analysis of MAL has contributed to our understanding of the evolution of multigenic families, the global integration of carbohydrate metabolism, and gene regulation.