Although present since the mid 19th century, the introduced snail species Cepaea nemoralis has spread extensively in Göteborg, mainly in the last two decades. Early samples show the predominance of yellow-shelled morphs, but in modern populations pink shells predominate. There is great variation in morph frequencies among modern populations with high values of F ST , and with no relationships to habitat. The patterns shown resemble those seen in Sheffield (UK), a city also colonised over a similar time period, but differ from those seen in regions where the species has been established for much longer. The combination of early and recent records suggests not only that founder effects play a large part in determining morph frequencies, but that present populations derive from multiple colonisations from different sources.