Using 150,000 actual trades, we examine competition among wholesalers, to which brokers route orders from their retail investors. We find that each broker's wholesaler execution prices have substantial dispersion and are persistent. However, most brokers do not adjust their routing even when they are sending more orders to higher-cost wholesalers. We also observe that the entry of a new wholesaler improves other wholesalers' execution quality. Finally, we present a stylized model that illustrates how brokers’ limited response to execution allows wholesalers to exercise their market power. Overall, our findings are inconsistent with perfect competition.