AbstractMitochondrial DNA (mtDNA) from 25 blue titsParus caeruleussampled from two populations of the Grenoble region (France) was assayed for polymorphism with 17 restriction endonucleases. Nine genotypes were found. Several mtDNA genotypes were also analysed by amplificationviathe polymerase chain reaction (PCR) and direct sequencing of 903 bp of the cytochromebgene. The mtDNA polymorphism is greater in P.caeruleusthan in other comparable bird species and results from the presence of two clearly differentiated mitochondrial lineages. Using the data of restriction polymorphism, the mean sequence divergence between individuals of the two lineages is 1.23%. Therefore,P. caeruleusshould fall into the category II of phylogeographic patternsensuAviseet al.(1987): discontinuous mtDNA genotypes which co–occur in the same region. P.caeruleus, like humans and other mobile species with high gene flow, seems to have lost its geographic structure in terms of mtDNA phylogeny. This unusual mitochondrial polymorphism can be explained by the recent admixture of two long–term isolated populations. This could be accounted for by two different scenarios. One assumes a simultaneous post–glacial colonization of the Grenoble region by two isolated European populations of P.caeruleus.Alternatively, hybridization between P.caeruleusand P.cyanuscould have caused the observed pattern of mtDNA vari