The Amy-locus polymorphism of Drosophila subobscura is used as a model-system for an experimental population genetic study of adaptive significance of alpha-amylase activity on substrates of different carbohydrate compositions. So far, fitness components have not commonly been included in ecological-genetic studies of alpha-amylase polymorphism in this species. In the present paper fitness components are analyzed in relation to different amylase activities in D. subobscura individuals homozygous for "slow" and "fast" Amy allele, associated with substrates of different carbohydrate compositions. The results indicate a significant effect of substrate carbohydrate composition on fitness components of the genotypes homozygous for S or F Amy allele in D. subobscura through their enzyme activity.