According to restriction site analyses of mitochondrial DNA, Drosophila subobscura shows a polymorphism that consists of two frequent haplotypes that are evenly distributed all over the Old World and several rare haplotypes never present in more than one locality. To ascertain the causes responsible for such distribution, three different mtDNA fragments from haplotypes I and II sampled in a population from Zürich have been partially sequenced. Only three silent nucleotide changes have been detected in the ND5 gene. One of them implies the loss of the HaeIII restriction site, which differentiates haplotype I from haplotype II. On the basis of these results as well as on others involving the geographic distribution of haplotypes I and II, they can be considered phenotypically equivalent. The sequencing study has been complemented with the analysis of cytonuclear disequilibria between mitochondrial haplotypes and different nuclear loci in four D. subobscura populations. As expected, no significant cytonuclear disequilibria have been found between haplotypes I and II. Moreover, when haplotypes I and II were pooled and compared with the rare, endemic haplotypes similar results were obtained.Key words: mitochondrial DNA, allozymes, selection, neutrality, cytonuclear disequilibria.