This study tested the assumption that a sense of collective identity stimulates participation in collective action. Contextual circumstances supposedly make a collective identity more salient and compel people to act as members of the group; protest participation is more likely among people with a strong collective identity. Group identification and participation in identity organizations were used as indicators of collective identity in a study of 248 farmers from Galicia (Spain) and 167 farmers from the Netherlands. The farmers were interviewed three times at intervals of 1 year. The longitudinal design also allowed a test of causality. A sense of collective identity appeared to stimulate preparedness to take part in farmers' protest. Action preparedness leads to action participation, which in turn appears to foster collective identity.