This thesis seeks to contribute to a literature which as yet barely exists in political science. It is a study of the transformation of a working-class political party, more specifically of the declining involvement in the New Zealand Labour Party of the organizations out of which the party grew, the trade unions. In the opening chapter, four indicators of the 'level' of trade union involvement in the party are examined. Changes in the proportion of all trade unionists and all trade unions affiliated to the party; in the proportion of trade unionists among all the delegates at, and in the proportion of union-originated remits considered by, the party's annual conferences; in the frequency of meetings of the Joint Council of Labour; and in the proportion of trade unionists among the members of the PLP all reveal that the Labour Party is much less the party of the organized working class today that it was in the years before World War II. In the subsequent chapters, the causes of the trade unions' gradual abandonment of the party are identified and discussed: the success with which they have been able to pursue their primary objectives under conditions of full employment in the post-war period; the institutionalization of the FOL, which has become an increasingly independent and effective interest group; the changing composition of the trade union movement; and the growing domination of the PLP by its middle-class elements. The conclusion states that, as a mass party, the Labour Party is moribund, but that, as the party of the working class, organized or otherwise, it is already dead.