In order to participate, to aspire to govern, and avoid to alienate potential voters, once marginal parties may become institutionalized and change their agendas and strategies. In the aftermath of the fiftieth anniversary of the Parti Québécois, what conclusions can be drawn from that organization ? This article explores in a comparative perspective the reasons behind the process of institutionalization, also studying the cases of the Scottish National Party in Scotland and the Sinn Féin in Northern Ireland. The data used are theoretical and empirical, based on the analysis of their electoral platforms. This allows to assess their antisystemic character, while considering their respective social and political contexts.