An optimal U.S. biodiesel fuel subsidy

► Taking price response into account, the optimal theoretical 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. ► 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.