Semilimax pyrenaicus has been known in Ireland for over 100 years, but not previously found in Britain, despite widespread recording of two other vitrinid species. Through fieldwork and the online recording system iRecord, large S. pyrenaicus populations were found in two river valleys, one in the South Wales Valleys (the Cynon Valley) the other in Snowdonia / Eryri National Park (Ceunant Cynfal). Though climatically similar, the two have very different histories, the Cynon Valley being a broad valley heavily deforested and modified by industry and settlement, and Ceunant Cynfal being a narrow gorge with remnants of ancient Atlantic Oak Woodland. In Britain, S. pyrenaicus appears to be a recent arrival (earliest record 2019) and seems likely to spread, perhaps aided by passive dispersal along rivers.