This study uses a unique firm-level dataset to examine how falling trade costs from 1993-2001 affected entry, exit, productivity, and exporting in the Korean manufacturing sector. We verify many of the predictions of recent heterogeneous-firm models of international trade. For example, falling trade costs reduced entry by new Korean firms, increased their probability of exit, and reduced the market share of surviving firms. We also find that small firms had a particularly high level of dynamism over the sample period. Small firms were more likely to enter and exit, and marginally more likely to gain market share, enter export markets for the first time, and improve their productivity.