We investigate, using firm level data for 79 developed and developing countries, whether differences in the allocation of resources across heterogeneous plants are a significant determinant of cross-country differences in income per worker. For this purpose, we use a standard version of the neoclassical growth model augmented to incorporate monopolistic competition among heterogeneous firms. For our preferred calibration, the model explains 58% of the log variance of income per worker. This figure should be compared to the 42% success rate of the usual model.