Many studies have documented that the sales concentration of U.S. producers has risen in recent decades. In this paper, we show that this increase was accompanied by more entry and growth of foreign competitors. Using confidential census data covering the universe of all firm sales in the U.S. manufacturing sector, we find that rising import competition increased concentration among U.S. firms by reallocating sales from smaller to larger U.S. firms and by causing firm exit. However, this increase in production concentration was counteracted by the expansion of foreign firms, which reduced domestic firms’ share of the U.S. market inclusive of foreign firms’ sales. We find that once the sales of foreign exporters are taken into account, U.S. market concentration in manufacturing was stable between 1992 and 2012.