Two butterfly species living in the Alps, Parnassius apollo and P. phoebus, frequently hybridize in certain localities of this region. The features of this phenomenon have been previously studied by biometry and starch gel electrophoresis, but some points remained obscure. We present them in a study combining results from cellulose acetate electrophoresis and wing pattern biometry with a determination of the mitochondrial haplotype by a PCR-RFLP analysis in a sample of butterflies from the southern French Alps. It was already known that the male hybrids are fecund and thus that interspecific gene exchange could take place via backcrosses with the parent species. In the present case, combining the identification of mtDNA with the analysis of nuclear genotypes allows us to demonstrate that hybridization can involve both sexes of both species. Moreover, it suggests that at least some female hybrids are not sterile. The impact of Haldane's rule is therefore not very strong in the present case. However, although the prerequisites for introgression between the concerned species are fulfilled, at the level of both nuclear and mitochondrial genomes, no indication of such a phenomenon could be gathered in the studied sample.