With the opening of the Mexican economy under the North American Free Trade Agreement (NAFTA), Mexican agriculture came under new competitive pressures from U.S. exports. It was widely recognized at the beginning of NAFTA that Mexico had geographically-based comparative advantages in supplying off-season fruits and vegetables to a hungry U.S. market. NAFTA’s liberalization of agricultural trade produced the expected results, with more staple crops and meats flowing south and more seasonal fruits and vegetables flowing north. In agriculture, tariffs and quotas have now mostly been eliminated. Not so agricultural subsidies, which were left largely undisciplined by NAFTA. High U.S. farm subsidies for exported crops, which compete with Mexican products, have prompted charges that the level playing field NAFTA was supposed to create is in fact tilted heavily in favor of the United States. This paper assesses the costs of U.S. agricultural policies to Mexican producers by examining the extent to which the United States exported agricultural products to Mexico at prices below their costs of production, one of the definitions of “dumping” in the WTO. We study eight agricultural goods – corn, soybeans, wheat, rice, cotton, beef, pork, and poultry – all of which are heavily supported by the U.S. government, were produced in Mexico in significant volumes before NAFTA, and experienced dramatic increases in U.S. exports to Mexico after the agreement. We look at the years 1997-2005 because the beginning year follows both the implementation of NAFTA and the enactment of the 1996 U.S. Farm Bill, which significantly changed the nature of U.S. farm support. We estimate “dumping margins” and the costs to Mexican producers of prices driven below production costs by U.S. policies. We estimate Mexican losses for the eight products at $12.8 billion over the nine-year period, more than the value of Mexican tomato exports to the United States. Corn farmers experienced the greatest losses: $6.6 billion, an average of $99 per hectare per year.