EXHIBIT 21. LIST OF SIGNIFICANT SUBSIDIARIES The following subsidiaries were 100 percent owned and were consolidated by the Corporation at December 31, 1999: STATE OR JURISDICTION NAME OF SUBSIDIARY IN WHICH INCORPORATED Marathon Canada Limited Canada Marathon Guaranty Corporation Delaware Marathon International Oil Company Delaware Marathon International Petroleum Ireland Limited Cayman Islands Marathon Oil Company Ohio Marathon Oil U.K., Ltd. Delaware Marathon Petroleum Company (Norway) Delaware Marathon Petroleum Investment, Ltd. Delaware Marathon Sakhalin Limited Cayman Islands United States Steel International, Inc. New Jersey U.S. Steel Mining Company, LLC Delaware USX Capital LLC Turks & Caicos Islands USX Capital Trust I Delaware USX Engineers and Consultants, Inc. Delaware USX Portfolio Delaware, Inc. Delaware The following subsidiary was 62 percent owned and was consolidated by the Corporation at December 31, 1999: Marathon Ashland Petroleum LLC Delaware