This article examines the relationship between Durban and Southampton constructed by the Union Castle Line between 1900 and the 1930s. It shows how specific, long-lasting patterns of commercial organisation and labour recruitment were laid down, and how they survived the contingencies of war, working class insurgency and financial crisis. The article proposes the concepts of ‘maritime capital field’ and ‘maritime labour field’ to describe the long-lasting shapes which transnational structural relationships gave to imperial shipping enterprises. A critique is made of the work of Michael Miller on European shipping companies in this period: the article demonstrates that the Union Castle case challenges both Miller’s emphasis on an expansive globalisation and his emphasis on the cosmopolitanism of the shipping industry. Political and social closures and limitations were characteristic of the relations between the two ports. Restrictive forms of political, ideological and military power within the British Empire played a great role in structuring the connections between Durban and Southampton.