Stone loach, Barbatula barbatula, occurs in scattered localities in Sweden. Some of thepopulations have usually been considered as feral descendants of escaped 18thCentury pond stock, but historical documentation is inconclusive. Using the mitochondrialCOI gene as a marker, we analyzed specimens from seven Swedish localities. Oneof the middle Swedish localities, in Stockholm, belongs to a haplotype found also inPoland and Lithuania. Two other samples, from near Nyköping and Lake Hjälmaren,belong to a haplotype found in northeastern Europe (Finland, Estonia, Latvia, Volgabasin in Russia). Those two Swedish populations are probably descendants from atleast two introductions, probably for pond rearing for human consumption. Samplesfrom Skåne and Halland in southern Sweden belong to the haplotype found inDenmark, northern Germany and Poland; and whereas it remains possible that theyalso represent feral populations, they may be naturally occurring, having reachedSweden during the Ancylus period, about 8,000–10,000 years ago. A recently discoveredpopulation from the central South Swedish Highlands belongs to a mainly southeastern European haplotype. It probably represents a release of imported aquariumspecimens or live bait carried by sport fishing tourists.