IT has been argued1 that natural selection acting on enzyme polymorphisms may be most clearly demonstrated by challenging polymorphic populations with an environmental additive acting as a specific substrate for the enzyme under study. Wills and Nichols2, for example, showed that, in their experimental conditions, heterozygote advantage at the octanol dehydrogenase locus in D. pseudoobscura depended on the presence of octanol in the food medium. Similarly, de Jong et al.3 found that the amylase4,6 variant increased strikingly in frequency in populations of D. melanogaster moved from a sucrose to a starch-rich food medium. Amy 4,6 is known to possess the highest in vitro activity on starch substrates of all common amylase variants4. We have examined the relationship between mortality and genotype at the alcohol dehydrogenase (Adh) locus when adult Drosophila are exposed to environmental ethanol.