The paper analyzes the impact of skill-biased migration policies under the economics of agglomeration. It therefore develops an agglomeration model with two types of mobile worker who are heterogeneous and differ both within and between skill groups with respect to their migration propensity. On the one hand, the model reveals that the effectiveness of migration policies depends on the level on trade costs. On the other hand, it shows that increasing (reducing) political barriers to migration for one factor of production, reduces (increases) the migration incentive of the other. Consequently, pro-skilled and contra-unskilled migration policies attenuate each other or can even be counterproductive.