I study the firm-level dynamic response of a commodity-exporting economy to global cycles in commodity prices. I propose a heterogeneous-firms model endogenizing manufacturing productivity slowdowns through reallocation towards less productive firms. Within a given sector, commodity booms reallocate market share away from exporters because of exchange rate appreciation and away from capital-intensive firms because of the increase in the required rate of return to capital. Using microdata for Chile, the largest copper producer worldwide, I provide empirical evidence for these channels. When fed with the commodity super-cycle of 2004–2012, the calibrated model generates between 13% to 32% of the observed productivity slowdown.