Translocation experiments can be used to study the factors limiting species' distributions and to infer potential drivers of successful colonisation during range shifts. To study the expansion dynamics of the butterfly Pyrgus armoricanus in southern Sweden and to find out whether its distribution was limited by climate, translocation experiments were carried out within and 50-60 km beyond its natural range margin. Populations were monitored for 8 years following the translocation. Although most translocation attempts failed, P. armoricanus was able to survive in two sites north of its current range limit. One of them eventually led to expansion and establishment of a viable metapopulation. Translocation success appeared to be independent of latitude, suggesting that climate is not the main factor determining the current northern distribution limits of this butterfly. Population growth and secondary spread in the expanding population were positively related to patch area and connectivity, while local habitat quality seemed to be less important. 5. The successful translocation and the importance of a well-connected patch network suggest that the current distribution of P. armoricanus is limited by its low dispersal ability combined with the fragmentation of its habitat, making it unlikely to track its changing climatic niche. Assisted migration could be an effective tool for such species, but long-term evidence for its effectiveness is not yet available.