W hen a shipper urgently needs truckload service, they often utilize the spot market. But despite its importance, little is known about this market. I analyze a longitudinal data set of auctions for spot truckload service where I observe invitations for carriers to bid, whether a bid is placed, and the bid price if one is placed, and augment this with information about the bidding carriers. Drawing upon auction theory, I suggest that a carrier's a priori characteristics (size, market specialization) explain the two primary decisions made in the auctions-whether to bid and how much to bid. I find that brokers, who act as intermediaries between shippers and asset-based carriers, bid much more frequently and higher than asset-based carriers. Price indexes show that broker bid prices follow similar patterns, but asset-based carrier prices do not. The results suggest that an online marketplace linking shippers directly with the thousands of asset-based carriers could add considerable value to the for-hire trucking industry, a development which appears to be happening.