We discuss the validity of Rybczynski’s theorem under increasing marginal costs within firms or industries. In particular, we show that an extra supply of any factor may lead to an expansion of all sector outputs if at least one sector permits input substitution. We provide a corresponding necessary and sufficient condition. This condition can even be satisfied when the equilibrium is Walrasian and Marshallian stable. Our findings are also robust with respect to aggregate improvements in total factor productivity which raise the economy’s outputs beyond private returns.