當廠商生產最終財必須相互使用到對方所生產的中間財，這樣的關係稱之為共生。本文即討論共生結構的兩個重直整合廠商的行為，以及本國政府面對此一廠商結構如何採取最適的貿易政策。本文以Spencer and Jones（1991）的模型為基礎設立一個兩國模型，本國與外國各擁有一個重直整合廠商。本國廠商與外國廠商各自生產一種中間財，並相互出口到對方以生產最終財。最後本國與外國廠商生產的最終財皆在外國最終財市場銷售。本國政府則對本國廠商，在中間財出口及最終財出口均採取貿易政策。 本文分別討論廠商面對最終財市場分別為Cournot或者Bertrand競爭之下本國政府將採取的最適貿易政策。最終財市場為Cournot競爭中，我們發現政府不是對本國廠商進行中間財出口補貼和最終財出口課稅，就是對本國廠商進行中間財出口課稅及最終財出口補貼。但在最終財市場為Bertrand競爭之下，本國政府則同時對中間財出口及最終財出口補貼，或是對中間財出口及最終財出口課稅。 The final goods production which are made by the firms who use each other intermediate inputs are called symbiotic production. This paper considers the optimal trade policies under two vertically-integrated firms with symbiotic production. We build up a two-country model based on Spencer and Jones（1991）. Both countries own a vertically-integrated firm. Both two firms produce one kind of intermediate input and use each other intermediate input for producing their final goods. All the final goods of the foreign and the domestic firms are consumed in the foreign final goods market. The domestic government adopts the optimal trade policy to its intermediate input as well as final products exports. This paper considers the optimal trade policy under either Cournot or Bertrand competition in the final goods market. Under Cournot competition, the optimal policy by the domestic country may require an export subsidy on intermediate inputs and an export tax on final products, or an export tax on intermediate inputs and an export subsidy on final products, depending on some specific conditions. On the other hand, under Bertrand competition, the optimal policy may require an export tax on both exports, or an export subsidy on both exports, depending on some specific conditions as well.
