Statistical analysis indicated that mean alcohol dehydrogenase (ADH) activity levels within ADH1 and ADH11 isochromosomal lines varied significantly among the Mexican populations ofDrosophila melanogaster used. Also as indicated by coefficient of variation values for ADH activity, the relative amount of genetic variation differed among and within the ADH1 and ADH11 strains examined. The quantitative nature of the differences in ADH activity supported the concept of the additive gene action of the alcohol dehydrogenase (Adh) locus. The manner in which our isochromosomal lines were constructed suggested that genes regulating ADH activity inD. melanogaster were located on the second chromosomes.