This paper takes issue with the common practice by economists of dismissing multiplant scale economies as vague or absent. After constructing an analytical framework for studying economies of scale in a multiple-plant setting, an optimization approach is used to quantitatively estimate multiplant scale economies of a particular type in two industries. Also developed are estimates of economies of scale from firm size, properly imbedded in a multiplant analysis. The empirical results yield some interesting observations on the complexity of the concept of optimal plant size.