This study explores the contribution of Freemasonry to the wider social, political and civic history of the South Wales town of Merthyr Tydfil during the nineteenth and early twentieth centuries. Merthyr has been chosen for a case of the local importance of Freemasonry because during this period it experienced considerable social, political and cultural changes brought about by intensive industrialisation and urban expansion, the town becoming a magnet for immigration and spawning a flourishing popular culture that had a dramatic impact on both the town itself and the Welsh nation more generally. The history of Merthyr continues to attract many Welsh historians. However, in the main they have concentrated on the development of the working class, emphasising the growth of a distinctive Welsh radical ethos and working class identity. This has been privileged over the equally important development of the town's middle class, which from the earliest stage of industrial development, was essential to sustain the needs ofa swiftly developing community. The growth of Merthyr's nascent middle class was intimately linked to the development of Freemasonry within the town, and the membership records of Masonic lodges are a rich source for reconstructing and investigating historical patterns of social, political and business networking within the community. This study explores the ways in which the membership records of Merthyr's oldest Masonic lodge, the Loyal Cambrian Lodge, can be used to reconstruct social networks and investigate the history of Merthyr' s middle class, thereby illustrating the pivotal role of Freemasonry in Merthyr during this period. Such analysis provides new perspectives on Merthyr's social history and suggests that the civic culture of Merthyr, of which the Masonic lodge was an integral part, was more sophisticated during the nineteenth century than previously supposed.