This article deploys comparative historical data on 14 OECD countries to examine the significance of predominant union structure for the impact of union strength on the (medium-term) growth in hourly labour productivity in manufacturing. The analysis shows that where craft and general unionism predominates, union strength has a deleterious impact on productivity growth. Where enterprise unionism predominates, union strength is irrelevant. However, where industrial unionism predominates, union strength promotes productivity growth. These effects exist independently of established economic influences on aggregate productivity growth. The findings are interpreted as displaying the importance of the character of the governance that unions provide for their productivity impact.