The optimal taxation of foreign and domestic investors' incomes is examined with a simple overlapping-generations model. Even when tax rates are allowed to discriminate between these groups, the optimal tax rates on both domestic and foreign investors' incomes in the small open economy are identical and equal to the optimal rate of tax in the closed economy. In light of the emphasis in the literature on the extent to which the elasticity of international flows might lower optimal capital income taxes, this conclusion is quite a surprise. In the large open economy, the optimal tax rate on foreign investors'income alone is a weighted average of one and the small economy tax rate. The optimal tax rate on domestic income is, again, unaffected by the openness ofthe economy. When a uniform tax rate must be set in the large open economy, it is generally higher than the optimal tax rate for a closed economy, a conclusion contrary to the conventional wisdom. However, a higher elasticity of international capital flows is associated with a lower tax rate, as expected, butthe rate remains above the closed-economy rate. In summary, openness matters for optimal tax policy, primarily in the case of the large economy. The reason is mainly the ability to burden foreign investors with a tax liability