This study examines member mobilization using a British community organization and a British trade union as exemplars. Although there has been substantial work on union revitalization on the one hand, and the emergence of alternative, community organizations on the other, no study has compared the challenges these organizations face, in encouraging member mobilization. The focus is on member commitment and the organization's legitimacy: the trade union gains merely pragmatic legitimacy from its members and the relationship between the member and the union is mainly instrumental, based on a negotiated exchange. The community organization, in contrast, gains moral legitimacy from its members and exemplifies a productive exchange between the members and the group.