Abstract
Laboratory populations of Drosophila melanogaster were subjected to heat and cold shock of sufficient duration and intensity to cause about 90 percent mortality. Alcohol dehydrogenase phenotypes were determined in survivors and controls. Significant changes in phenotypic frequencies occurred as a result of both treatments such that relative fitness appeared to be dependent on temperature. The alcohol dehydrogenase phenotype most frequently surviving heat under laboratory conditions is the same phenotype as that previously found to be higher in frequency in natural populations in areas with higher average temperature.