This paper begins by reviewing the recent literature on the labour market benefits of citizenship acquisition among immigrants to Canada and the United States, and goes on to ask why a gap in the citizenship rate among immigrants developed between Canada and the United States in the period from 1970 to 2006. This paper then examines the impact of demographic changes among immigrants to Canada and the United Sates on the evolution of this citizenship rate gap and discusses the potential impact of other factors.--Document.