Ethanol levels in Drosophila breeding sites were higher in a winery storing fortified wines than in nearby grape pressings or in orchard fruits. The relative abundance of D. simulans to D. melanogaster was negatively correlated with ethanol levels. In D. melanogaster there were no significant differences in AdhF frequency between the orchard and winery populations. The ethanol tolerance of wild caught D. melanogaster males paralleled the levels of ethanol in the breeding sites but Adh alleles and ethanol tolerance segregated largely independently of each other. Levels of ADH activity were positively associated with the ethanol tolerance of the different populations and with levels of ethanol in the breeding sites, but it is argued that the ethanol levels are not causative. Flies from inside the winery had higher ADH levels due mainly to greater amounts of ADH-F. The difference in activity persisted for at least one generation in the laboratory. After ten generations of laboratory culture the differences in ethanol tolerance were still present but there were no significant differences in ADH activity.