Cuticular hydrocarbons of youngDrosophila flies are singular with very long chains and complex diene mixtures. A precise characterization of these substances was carried out by epoxidation and analysis of the products by GC-MS with negative chemical ionization. InD. melanogaster, double bonds of dienes are more probable at carbon positions 11 or 13 and 21 or 23. InD. simulans, double bonds are shifted more towards the interior of the chain. Such a difference is also found among monoenes of both species. The analyses of monoenes and dienes confirm the similarity of cuticular compounds of young flies of both sexes in both species. A main cuticular compound ofD. erecta females, 9, 23-tritriacontadiene, is also presented.