This paper uses data on US manufacturing firms to study a new extensive margin, the reallocation of resources that takes place within surviving firms as they open and close establishments in different regions. To motivate the empirical analysis, I extend existing models of industry dynamics to include production-location decisions within firms. The empirical results provide support for the mechanisms emphasized by the theoretical model. In the data, only about 3 percent of firms make the same product in more than one region, but these multiregional firms are more productive on average compared to single-region firms, and they account for about two-thirds of output. The results also show that "region-switching" is pervasive among multiregional firms, is correlated with changes in firm characteristics, and leads to a more efficient allocation of resources within firms.