Enhanced environmental quality, fuel security, and economic development, along with reduced prices of blended diesel, are often used as justifications for a U.S. federal excise tax exemption on biodiesel fuels. However, the possible effect of increased overall consumption of fuel in response to lower total prices, mitigating the environmental and fuel security benefits, are generally not considered. Taking this price response into account, the optimal U.S biodiesel subsidy is derived. Estimated values of the optimal subsidy are close to the recently expired subsidy, revealing the subsidy's environmental and security benefits. However, further positive environmental and security benefits from the biodiesel tax-exemption subsidy may be obtained if the subsidy is combined with a federal excise tax on petroleum diesel. (C) 2012 Elsevier Ltd. All rights reserved.