Long Island Sound is plagued by conditions of severe hypoxia (low levels of dissolved oxygen) during the summer months because of the existence of excessive amounts of nitrogen. A new proposal that would allow sewage treatment plants to buy or sell nitrogen discharge credits is currently being evaluated by the states of Connecticut and New York. Existing theory suggests that a trading program for nitrogen emissions would be a cost-effective means of addressing the problem. We estimate the costs associated with several trading scenarios and find that the potential for cost savings is substantial and that cost savings rise as the scope of trading expands. ¿ 2000 American Geophysical Union