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Most of Switzerland is inhabited by the nominotypical subspecies of the barred grass snake (Natrix helvetica helvetica), which is characterized by mitochondrial DNA lineage E. Only in the northeast of the country, the common grass snake (N. natrix) occurs and hybridizes with N. h. helvetica in a narrow contact zone. However, we discovered that in southern and western Switzerland barred grass snakes representing another mtDNA lineage (lineage C) are widely distributed. Lineage C is typical for Alpine populations of the southern subspecies N. h. sicula. Our microsatellite analyses of the Swiss samples revealed differences between the two subspecies and also a substructure with two clusters in each subspecies. Furthermore, we discovered a contact and hybrid zone of N. h. helvetica and N. h. sicula along the northern shore of Lake Geneva and also confirm that interbreeding with alien common grass snakes (N. n. moreotica, mtDNA lineage 7) occurs there. This finding is of concern for nature conservation and measures should be taken to prevent further genetic pollution. Using morphometrics, we found no differences between the two subspecies of N. helvetica, while N. natrix was slightly distinct from N. helvetica. [ABSTRACT FROM AUTHOR]