This paper examines empirically the relationship between vertical integration and wholesale gasoline prices. We use discrete and differential changes in the extent of vertical integration generated by mergers in West Coast gasoline refining and retailing markets to test for incentives to raise rivals' costs. The research design allows us to test for a relationship between vertical integration and wholesale prices, controlling for horizontal market structure, cost shocks and trends. We find evidence consistent with the strategic incentive to raise competitors' input costs. This suggests that vertical integration can have a significant impact on wholesale prices.