We analyze the price relationship of refined-bleached-deodorized (RBD) soybean oil prices among four regional U.S. markets (Central Illinois, U.S. Gulf, West Coast, and East Coast). Econometric time-series methods were used to detect price integration, linkages, and responsiveness for each oil type and among each market. Results show that the four markets have remained price-integrated in the long run. This implies that the markets are spatially efficient. The results, however, also suggest that the level of market efficiency may have decreased to some extent after the U.S. biodiesel production surge in the mid-2000s.