Despite prominent examples of anti-competitive practices by employers, collusion has received limited attention in research on the sources of labor market power. We study collusive wage-setting behavior by 227 coal firms in Belgium from 1845-1913, a setting in which collusive wage-setting was legally tolerated, whereas worker collusion was forbidden. We propose an empirical framework to identify collusion in labor markets using production, cost, and wage data. We find evidence for substantial collusion against workers, which is consistent with anecdotal evidence. Collusion was stable throughout Belgium’s Industrial Revolution from 1845 to 1900, but increased sharply after the turn of the century. This surge in collusion coincided with the emergence of coal cartels.