Using research into the formation of industrial populations in the nineteenth century, this article examines the relationships between immigration and natality in Tilleur, an exemplary locality for studying the industrial revolution in Belgium. The main purpose is to test the general hypothesis positing a distinction between a foundation phase and a maturation phase in the process through which an industrial population is formed. The results are a contribution to the debate about the beginning of the fertility transition in industrial cities, and its relations to differential nuptiality and fertility in light of spatial origins.