This article examines the conditions under which, in June 1975, a marginalized, stigmatized population -the prostitutes of the French city of Lyon- took the step of collective action, occupying a church for more than a week to protest against police repression. It highlights the difficulties these politically inexperienced women encountered in mobilizing, namely preventing defections and choosing an appropriate mode of action ; difficulties they were able to surmount thanks to resources provided by outside supporters endowed with practical knowledge in matters of collective action (activists in a group with close ties to the broad movement known as catholicisme social and feminists). Despite this assistance, however, the prostitutes' mobili...