The Zaanstreek—northwest of Amsterdam, The Netherlands—has been a highly industrialized region for nearly four hundred years. For most of this period, it showed a strong sense of community and a high degree of cooperation between firms, which is usually considered to be typical for an “industrial district.” However, between about 1840 and the First World War the character of this industrial district was dramatically transformed. In response to the rapidly growing integration of markets in the national and international economies since the 1840s, the Zaanstreek went through a radical change in energy base, as well as a fundamental shift in industrial structure. This essay addresses the questions of what happened to interfirm cooperation in the Zaanstreek when this fundamental transition in energy base and shift in industrial structure came about and how and to what extent entrepreneurs in the Zaan district between 1840 and 1920 managed to preserve the sense of community and interfirm cooperation that were the hallmarks of this region since the seventeenth century. It shows that the principle of mutuality eventually proved no longer strong enough to keep the actors in these institutions together. Ties between firms within the industrial district were in several respects replaced by, or subordinated to, ties between firms outside the industrial district. What kept interfirm cooperation in the district nevertheless intact for much of the period after 1920 were ties of regional family networks. The essay concludes with a few observations about the relevance to the study of industrial districts in general.