Before 1978, most of the domestic copper production in the US and an important share of imports were traded at a price set by the major US producers. At the same time, the rest of the world was trading copper at prices determined in auction markets. This two-price system ended in 1978, when the largest US producers began using the Comex price of refined copper as a benchmark for setting their prices. Using this regime shift, I empirically test the competitive behavior of the US copper industry before 1978. The results show that copper prices were close to the levels predicted by a competitive model of the industry.