We investigate whether taxpayers respond to a large, ACA-induced upward tax notch at the Federal Poverty Level. Using administrative tax data, we document bunching in the income distribution and provide estimates of the taxable income elasticity. Our estimates are the only ones using US data that are identified by panel variation in budget constraints rather than functional form assumptions (Blomquist and Newey 2017), a feature of our novel longitudinal estimator. Consistent with Saez (2010), we find bunching only among the self-employed. Analysis of linked tax and survey data suggests that bunching is likely not representative of changes in labor supply.