Between c.1840 and the First World War, the industrial district of the Zaanstreek, The Netherlands, underwent a radical change in energy base—from windpower to steam—and a fundamental shift in industrial structure. This paper focuses on the change in forms of inter-firm cooperation that accompanied this drastic transformation. The specific issue that the paper seeks to address is: How and to what extent did entrepreneurs in the Zaanstreek maintain or adapt the institutions in which inter-firm co-operation took shape? The case study shows why institutions of inter-firm cooperation based on the principle of mutuality, which had served as key elements of this industrial district for over 150 years, did not survive the transformation at the end of the nineteenth century, and why new formal institutions of inter-firm cooperation in the end failed to take their place. 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 inter-firm cooperation in the Zaanstreek nevertheless intact until the 1950s were the ties of regional family networks. But the paper shows that these networks did not remain immune to the effects of the industrial transformation between 1840 and 1920.