In a model where a monopolistic downstream firm (assembler) negotiates simultaneously with each of its n subcontractors the prices of the complementary components which enter its product, we show that backward integration is limited by a strategic negative effect: the prices and profits of independent suppliers increase when a merger reduces their number. Mergers are profitable only if the downstream firm buys at least two thirds of its suppliers. In an endogenous acquisition game à la Kamien and Zang (1990) the only merged equilibrium occurs when there is only one subcontractor. In a sequential acquisition game full integration is not an equilibrium when the number of suppliers is at least five.