This thesis examines the decline of the Labour party in two constituencies in northwest Wales (Caernarfonshire and Merioneth) from the mid 1960s onwards, a decline which culminated in the party's defeat at the hands of the Welsh Nationalist Party - Plaid Cymru - in the general election of February 1974. Whilst the study is essentially local, it places political change in the region within much broader experiences and circumstances. National as well as local forces for political change are examined. Social, economic and cultural factors underpinning those changes are also considered. The structure of the thesis is broadly chronological and is divided into six chapters. The first chapter examines Labour's attempt to embed itself within these unfamiliar, rural, traditionally Welsh speaking communities before and after the Second World War. It shows that after 1945, Labour's (overwhelming) electoral successes were based on a combination of local and national, cultural and linguistic as well as economic appeals. The second chapter examines social and economic change from 1960-74, and shows the socio-economic challenges faced by Labour in the 1960s and early 1970s. These changes were the result of numerous factors, both long term and short term. It shows how national trends such as `affluence' combined with local concerns such as depopulation and the decline of the Welsh language to present powerful political challenges. The third chapter examines Labour's response to these challenges. It shows that whilst Labour was still successful, its commitment and plans to deliver social and economic reforms were undermined by the economic problems of the mid to late 1960s. Chapter Four shows how a viable political challenger to Labour - Plaid Cymru - emerged in the 1960s. Chapter Five focuses on Plaid's efforts to undermine Labour's credibility in the 1960s and on Labour's response to the emerging nationalist challenge. Chapter Six focuses on the general elections of February and October 1974, on the problems facing Labour in developing `new' solutions to the regions problems and on the popularity of Plaid Cymru's appeal.