Online platforms can strongly influence consumer behavior through default options, creating incentives to steer consumers to their own products and services. In this paper, I examine how Amazon determines which merchant is the default option on the Amazon Buy Box. Using data on hundreds of thousands of products and several countries, I show that the Amazon platform substantially prioritizes its retail arm and fulfillment arm in the algorithm. Amazon’s preference for its fulfillment arm has much larger effects on the default platform price and share of third party merchants winning the Buy Box than its preference for its retail arm.