Determining the significance of hybridization events raises essential issues both in conservation and in evolutionary biology. Here, we report a genetic investigation of sympatric polecat and endangered European mink populations. Although the two species were morphologically very similar, the European mink and the polecat were easily discriminated from allozymes and microsatellites and showed a high level of private alleles (effective number of alleles: mink=1.45 and polecat=3.09). Nevertheless, the allozymic polymorphism remained lower in the European mink (4 loci, 10.5%) than in polecat (9 loci, 23.7%). Similarly, from microsatellite data, the polymorphism only reached 36% at 0.99 in the European mink; whereas in the polecat, the polymorphism reached 82% at 0.99. Natural hybridization events between two native species were detected. Because of the low fertility of hybrids, interbreeding could be regarded as producing "hybrid sink" that leads to a progressive assimilation of mink by polecat. Nonetheless, pure mink populations inhabited streams in western France, and hybridization events were only detected in areas where mink were rare and now presumed disappeared. Rather than revealing the poor efficiency of the specific recognition system, our results suggest that hybridization is associated with the scarcity of mating partners.