High frequencies of the fast allele of alcohol dehydrogenase–2 (Adh–2F) are found in populations of Drosophila mojavensis that inhabit the Baja California peninsula (race BII) whereas the slow allele (Adh–2S) predominates at most other localities within the species' geographic range. Race BII flies utilize necrotic tissue of pitaya agria cactus (Stenocereus gummosus) which contains high levels of 2–propanol, whereas flies from most other localities utilize different cactus hosts in which 2–propanol levels are low. To test if 2–propanol acts as a selective force on Adh–2 genotype, or whether some other yet undetermined genetic factor is responsible, mature males of D. mojavensis lines derived from the Grand Canyon (race A) and Santa Catalina Island (race C), each with individuals homozygous for Adh–2F and Adh–2S, were exposed to 2–propanol for 24 h and ADH–2 specific activity was then determined on each genotype. Flies from five other localities homozygous for either the fast or slow allele also were examined. Results for all reported races of D. mojavensis were obtained. 2–propanol exposure inhibited ADH–2 specific activity in both genotypes from all localities, but inhibition was significantly less in two populations of race BII flies homozygous for Adh–2F. When F/F and S/S genotypes in flies from the same locality were compared, both genotypes showed high 2–propanol inhibition that was not statistically different, indicating that the F/F genotype alone does not provide a benefit against the inhibitory effects of 2–propanol. ADH–1 activity in female ovaries was inhibited less by 2–propanol than ADH–2. These results do not support the hypothesis that 2–propanol acts as a selective factor favoring the Adh–2F allele. J. Exp. Zool. (Mol. Dev. Evol.) 304B:159–168, 2005. © 2005 Wiley-Liss, Inc.