Recent theoretical developments on multiproduct firm costs have increased our understanding of these firms. Yet, to date, empirical analysis of the relationship between multiproduct firm costs and industry structure has been limited. In this paper, the multiproduct nature of US energy producers is explicitly recognized, and a multiproduct cost function estimated for the industry. The empirical results indicate that the multiproduct structure of large energy firms has, at least in part, been motivated by economies of scope between petroleum and coal operations. 27 references, 19 footnotes, 3 figures, 1 table.