A concerted effort is under way to analyze, at the genetic, biochemical, and molecular level, the Adh gene system in the medfly Ceratitis capitata, an important agricultural pest. The isoelectric focusing (IEF) pattern of alcohol dehydrogenase (ADH) of the medfly demonstrates the presence of two well-differentiated, genetically independent dimeric proteins, called ADH-1 and ADH-2. These proteins do not exhibit interlocus heterodimeric isozymes, and the genes are not controlled coordinately during development, Adh1 and Adh2 being expressed mainly in muscle or in fat body and ovary, respectively. From the intensity of the IEF isozyme patterns, primary alcohols are judged to be better substrates than secondary alcohols, in contrast with Drosophila melanogaster ADH, and ethanol is probably the most efficient substrate for both sets of isozymes. The isoelectric points of ADH-1 (pI = 5.4) and ADH-2 (pI = 8.6) are different from D. melanogaster ADH (pI = 7.6), but the medfly ADH-1 has a native molecular weight (approx. 58 kD) close to that of D. melanogaster. A population survey of samples both from laboratory strains and from wild geographically different populations showed that the Adh1 locus is more polymorphic than Adh2. The most variable populations are from Africa, the supposed source area of the species. Further, a case of selection at the Adh1 locus under laboratory conditions is reported. The hypothesis of Adh gene duplication and the degree of similarity between medfly and Drosophila ADH are also discussed.