Recent Spanish tax reform granted regions the authority to set income tax rates, resulting in substantial tax di erentials. We use individual-level information from Social Security records over a period of one decade. Conditional on moving, taxes have a significant effect on location choice. A one percent increase in the net of tax rate for a region relative to others increases the probability of moving to that region by 1.7 percentage points. Focusing on the stock of top-taxpayers, we estimate an elasticity of the number of top taxpayers with respect to net-of-tax rates of 0.85. Using this elasticity, a theoretical model implies that the mechanical increase in tax revenue due to higher tax rates is larger than the loss in tax revenue from the out-flow of migration.