Perhaps by virtue of its theoretical slipperiness, collective identity is often hailed as an important feature of social movements for the role it plays in unifying activists and organizations, and so helping them to develop shared concerns and engage in collective action. However, this paper argues that collective identity is the result of group rather than movement level processes, and although it can unite activists within a single movement organization, it is not always beneficial for the broader social movement. Although movements consist of networks of activists and organizations that have a broad shared concern, differing collective identities within the movement can actually be quite divisive. Based on case studies of three organizations in the environmental movement, this paper shows that activists who are most committed to an organization with an encompassing collective identity develop a strong sense of solidarity with other activists similarly committed to that organization. The resultant solidarity leads to the construction of a ‘we–them’ dichotomy between organizations within the same movement, increasing the chances of hostility between organizations and factions within the movement.