Support for the cooperative yardstick hypothesis was found using a standard structure-performance model that was extended to include a cooperative market share variable and was estimated with a large cross-section of food manufacturing markets. Market concentration and advertising intensity were positively related to price-cost margins. In addition, the aggregate market share of the one hundred largest agricultural marketing cooperatives was inversely related to price-cost margins. The magnitude of the effect was largest in the more concentrated markets. This suggests that. where cooperatives have vertically extended themselves into food processing. more competitive outcomes are found even in highly concentrated markets.