The role played by international migration and its interactions with other demographic components in the process of city growth remains underappreciated. We analyze trends in natural increase and in net internal and international migration in Zurich during the Industrial Revolution by relying on historical statistics disaggregated by citizenship and indirect demographic estimation techniques. Our results challenge demographic theories that attribute a predominant role to natural increase in city growth, finding that natural increase was positive only because of the migrants' contribution. Due to an early fertility decline, Zurich grew essentially through migration, with dramatic changes in the internal and international sources of migrants over time. Our discussion is centered in the context of structural changes in regional and international labor markets, and it further considers Zurich's discontinuous process of economic development and contrasting migration policies..