Cepaea nemoralis is an introduced species on Gotland. The first records, from the capital city of Visby, date from the 1860's. Known initially from only three localities, it remained uncommon and restricted through the first half of the 20th century. Sampling from 1992 onwards, however, revealed that the species had spread into many sites in and near the city and also to Ardre on the opposite side of the island. Populations from both the 19th century and from the last 27 years were polymorphic for both colour and banding patterns. There is no evidence of general and consistent change in morph frequencies over time. While the frequencies of various banding patterns vary in a manner attributable to genetic drift and founder effect, variation in the frequencies of colour morphs is more restricted, suggesting that selection may also be involved. The data here provide a baseline for monitoring future changes.