Studies of the isozymes produced by alternative alleles at the alcohol dehydro- genase locus of Drosophila melanogaster indicate that the ADH F enzyme is more active but less stable than the ADH S enzyme. The difference in stability is manfested in the responses to various conditions of temperature, pH, and protein concentration. The two enzymes also appear to differ in their substrate speeificities. It is clear that the differences of primary structure involved in the A DHpolymorphism can have profound effects on the biological activity of the molecule.