US Electricity and natural gas markets have traditionally been serviced by one of two market structures. In some markets, electricity and natural gas are sold by a regulated dual-product monopolist, while in other markets, electricity and natural gas are sold by separate regulated single-product monopolies. I analyze whether electricity and natural gas prices depend on the market structure and compare these results to the predictions of a number of theories. The results are most consistent with the political economy theories suggesting that regulators respond to interest group activity.