Membership mobilization is widely regarded as critical for union revitalization. Estimates of the level of activism vary widely, and studies reveal puzzling inconsistencies between union members' beliefs and intentions. Drawing from Ajzen's theory of planned behaviour, we address both issues using a sample of faculty union members. Results show both consistency and discrepancy between summary self-reports of activism and specific participation behaviours, helping account for the widely varying estimates of activism levels found in other studies. Results also indicate an important role for perceived control, a factor rarely examined in prior research on activism.