Our primary results indicate that incumbents in the airline industry do respond to the threat of entry. Incumbents drop average fares substantially when Southwest threatens a route (before Southwest actually starts flying the route). This is true even when we compare the fare changes on threatened routes to those on incumbents’ other routes out of the same airports, suggesting that shifts in airport-specific operating costs are not driving the results. The lower prices, in turn, appear to increase the number of passengers flying on the incumbents prior to entry. We also find, interestingly, that although incumbents cut fares on the directly threatened route, they do not cut prices on routes to nearby airports in the same market (e.g., no cuts on the Chicago-O’Hare route when Southwest threatens a Chicago-Midway route).