This paper contains estimates of the determinants of the commodity composition of U. S. trade flows and of price‐cost margins in U. S. domestic manufacturing in a simultaneous context. Above‐competitive profits in U. S. manufacturing appear to provide a powerful incentive to import competition. These imports, in turn, provide a check on the ability of domestic firms to earn monopoly rents. The estimates indicate that failure to account for imports leads to a substantial underestimation of the impact of market structure on price‐cost margins. The linkages between exports and domestic market conditions appear to be of second‐order imp