We study the marginal tax rate incorporated into short-term municipal rates using municipal swap market data. Using an affine model, we identify the marginal tax rate and the credit/liquidity spread in 1-week tax-exempt rates, as well as their associated risk premia. The marginal tax rate averages 38.0% and is related to stock, bond, and commodity returns. The tax risk premium is negative, consistent with the strong countercyclical nature of after-tax fixed-income cash flows. These results demonstrate that tax risk is a systematic asset pricing factor and help resolve the muni-bond puzzle.