The paper analyzes how US legacy carriers and Southwest Airlines respond to the threat of entry by AirTran Airways. It is found that that legacy carriers' ex post equilibrium prices are on average lower, whereas those of Southwest are on average higher in response to AirTran's threat of entry. Once AirTran actually flies the route, while the prices of legacy carriers fall further, those of Southwest don't change significantly. Hence, estimation results suggest that the pricing behavior of a low-cost incumbent is quite different from incumbent legacy carriers in response to not only actual but also potential competition by a low-cost carrier entrant. (C) 2012 Elsevier Ltd. All rights reserved.