Using bunching induced by a policy notch for identication, we rst provide evidence of investor inattention to a very simple and well-known capital-gains tax exemption in the Brazilian stock market. We then show that tax-inattentive investors exhibit stronger behavioral biases and worse trading performance, even after controlling for several investor-level variables such as trading experience, observed performance,nancial sophistication, age, and occupation. This is consistent with inattention being a common source of behavioral biases.