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A standard result in regulation is that efficiency requires that marginal prices be set equal to marginal costs. This paper performs an empirical test of marginal cost pricing in the natural gas distribution market in the United States during the period 1989-2008. For all 50 states we reject the null hypothesis of marginal cost pricing. Departures from marginal cost pricing are particularly severe in residential and commercial markets, where we find average markups of over 40%. Based on conservative estimates of the price elasticity of demand these distortions impose hundreds of millions of dollars of annual welfare loss. Moreover, the current pricing schedules are an important pre-existing distortion which should be taken into account when evaluating carbon taxes and other policies aimed at addressing external costs. Current markups for residential and commercial customers are already equivalent to a tax of over $200 per metric ton of carbon, considerably higher than the range envisioned by most economists.