Physico-chemical properties of ADH and some fitness parameters were examined in two mutant (cn and vg) and two wild-type (C-S and D) strains of Drosophila melanogaster. It was shown that, under the experimental conditions, longevity, fecundity and heat resistance did not depend on the activity and the electrophoretic mobility of enzymes. The Adh gene-enzyme system of the mutants was analyzed in relation to the saturation of their genotypes with genes of wild-type flies having different allelic control of the enzyme. ADH activity was shown to be positively correlated with the frequency of F allele of the structural gene (r = 0.84), whereas thermostability of the enzyme was not associated with electrophoretic mobility. Low thermostability of ADH in vg mutants, which was correlated with low heat resistance (r = 0.94), is assumed to be controlled by the thermostable allele Adh Fs.