This paper explores the nature of collective action and group behaviour through a case study of a highly successful political organization of poorer sex workers in Calcutta. The paper asks: What stimulated the participation of sex workers in their organization, promoted individual commitment to the group, and engendered co-operative action and group cohesion? What contributed to its success in achieving well being and equity, and in making itself dynamic and sustainable?In answering these questions, three main arguments are highlighted. First, the paper emphasizes collective self-representation and expression of community identity as the motor of group activity, rather than the gratification of individual material needs, or subjective personal satisfaction. The attempt here is to go beyond an understanding of group behaviour based on methodological individualism which seeks to explain the operation of groups in terms of the benefits that individuals within a group derive as individuals from their participation in a collective. The paper argues that individuals cohere in a group from a genuine belief in the normative superiority of the collectivity, even when little direct or immediate benefit accrues to each individual from participating in such a collectivity. Second, the paper underscores how collective action is propelled by political initiatives to reconfigure extant power relations of domination and subordination, and the struggle to interrogate or challenge established norms of social hierarchy and distribution. This is a largely under-emphasized theme in the literature and practice of development, where 'politics' is usually seen in terms of making claims or advocacy in the interest of disadvantaged groups. This paper shows that the success of the DMSC as a dynamic group lies in outstripping precisely such a limited conception of politics. Finally, this paper demonstrates how political activism enabled the recasting of identity, reconstitution of the 'self' and redefinition of subjectivity. It shows how sex workers as political actors came to reconceptualize their own potentials as human subjects, which in turn helped them to sustain collective action and enlarge its scope.