Dixit has recently presented a model in which established firms select capacity to discourage entry but cannot employ threats they would not rationally execute after entry. Entry deterrence in a slight modification of this model involves the classical limit-price output. Under linear or concave demand, however, the capital cost of a firm of minimum efficient scale is an upper bound on the present value of the monopoly profit stream that can be shielded from entry. It is argued that this suggests the general unimportance of entry barriers erected by scale economies.