This study measures the retail and wholesale price effects in Louisville, Kentucky resulting from the Marathon/Ashland (MAP) joint venture. MAP was an early transaction in the recent era of petroleum mergers and it caused sizeable changes in concentration. We find no evidence of increased retail prices resulting from MAP. Wholesale (rack) prices increased significantly approximately 15 months following the transaction. This wholesale price (rack) increase, however, was probably caused by a regional supply shock rather than the transaction. These results suggest in this case that a significant petroleum merger in a moderately concentrated market did not raise consumer prices.