We quantify agglomeration spillovers by estimating the impact of the opening of a large manufacturing plant on the total factor productivity (TFP) of incumbent plants in the same county. We use the location rankings of profit-maximizing firms to compare incumbent plants in the county where the new plant ultimately chose to locate (the “winning county”), with incumbent plants in the runner-up county (the “losing county”). Incumbent plants in winning and losing counties have similar trends in TFP in the seven years before the new plant opening. Five years after the new plant opening, TFP of incumbent plants in winning counties is 12% higher than TFP of incumbent plants in losing counties. Consistent with some theories of agglomeration economies, this effect is larger for incumbent plants that share similar labor and technology pools with the new plant. Consistent with a spatial equilibrium model, we find evidence of a relative increase in skill-adjusted labor costs in winning counties. This indicates that the ultimate effect on profits is smaller than the direct increase in productivity.