In a model where upstream network insiders conduct relationship-specific investment, downstream firms have an incentive to transact within networks. Evidence from US auto parts exports to 26 auto-producing countries supports key predictions of the model. Greater production scale for assemblers lowers imported parts per car. Vertical networks matter in two ways. First, although Japan’s average import levels are not unusually low, non-Japanese suppliers have relatively low market penetration for parts categories where vertical keiretsu are prominent in Japan. Second, US-owned assembly abroad and foreign-owned parts production in the US both stimulate parts exports.