Social identity theory is employed to conceptualise the role of group identi®cation in the conversion of discontent into participation in political protest. It is assumed that higher levels of group identi®cation stimulate participation in protest on behalf of the group. Perceived characteristics of the intergroup situation such as the permeability of group boundaries, and the stability and legitimacy of intergroup relations are supposed to modify the role of group identi®cation. Group identi®cation is decomposed into an aective and a behavioural component. Furthermore, ingroup identi®cation is distinguished from outgroup dierentiation; and groups are de®ned at dierent levels of inclusiveness. In a longitudinal study among Dutch farmers (n 168) the relationship between group identi®cation and protest participation is investigated. Group identi®cation, be it aective or behavioural, appears to in¯uence action preparedness. People seem to enter the protest arena with some level of group identi®cation. This level of group identi®cation sets the level of action preparedness. Once set, the level of action preparedness remains fairly stable over time and appears to be a strong predictor of future action preparedness and participation. Action preparedness in its turn together with the behavioural component of group identi®cation in¯uences actual participation in collective action. Outgroup dierentiation did not have any impact on protest participation. Identi®cation with farmers in the European Union did not matter, but identi®cation with farmers at the national or regional level did stimulate protest participation. Perceived characteristics of the intergroup situation did not have an impact on group identi®cation, but permeability and stability did aect protest participation. Theoretical implications of the ®ndings are discussed.