We investigate a private value auction in which a single "entrant" on winning imposes a negative externality on two "regular" bidders. In an English auction, when all bidders are active "regulars" free ride, exiting before price reaches their value. In a first-price sealed-bid auction incentives for free riding and aggressive bidding coexist, limiting free riding. We find substantial, though incomplete, free riding in the clock auction. In first-price auctions, regular bidders bid more aggressively than the "entrant" and both bid higher than in auctions with no externality. Predictions regarding revenue, efficiency, and successful entry between the two auctions are satisfied.