Social identity theory is employed to conceptualise the role of group identification in the conversion of discontent into participation in political protest. It is assumed that higher levels of group identification 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 identification. Group identification is decomposed into an affective and a behavioural component. Furthermore, ingroup identification is distinguished from outgroup differentiation; and groups are defined at different levels of inclusiveness. In a longitudinal study among Dutch farmers (n=168) the relationship between group identification and protest participation is investigated. Group identification, be it affective or behavioural, appears to influence action preparedness. People seem to enter the protest arena with some level of group identification. This level of group identification 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 identification influences actual participation in collective action. Outgroup differentiation did not have any impact on protest participation. Identification with farmers in the European Union did not matter, but identification 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 identification, but permeability and stability did affect protest participation. Theoretical implications of the findings are discussed. Copyright © 1999 John Wiley & Sons, Ltd.