The group of crested newts distributed from the southern Balkans to the southern shore of the Caspian Sea, Triturus karelinii sensu lato, comprises two species, T. karelinii in the east and T. arntzeni in the west. Three hypotheses have been forwarded defining the range of T. arntzeni, namely from northern Serbia eastwards i) in to Thrace, ii) up to the Aegean-Black Sea waterway including the Bosporus, or iii) into western Anatolia. We study 130 newts from 22 populations across this area with a panel of 40 enzyme nuclear genes. A combined analysis with the computer programs Structure and NewHybrids reveals the existence of two groups with admixture at two localities. The 'western group' comprises all European populations and a population at the southern shore of the Sea of Marmara in Asiatic Turkey, whereas the 'eastern group' is found at the Sakarya river valley in northern Anatolia, Asiatic Turkey. The admixed populations are also located in northern Anatolia. An analysis with the computer program BAPS resolves six genetic clusters, of which three represent the 'western group' and the other three coincide with the 'eastern group' and the two admixed populations. These analyses indicate that the species transition from T. arntzeni to T. karelinii is not in Thrace but in northern Anatolia. The presence of 'western' T. arntzeni to the east of 'eastern' T. karelinii indicates that the species' contact zone has a convoluted shape. Moreover, the spatial distribution of diagnostic allozymes only roughly coincides with that of two deeply divergent mitochondrial DNA haplotypes. A more detailed survey on the crested newt distribution in Anatolia is required to elucidate the picture further.