This paper analyzes the bidding behavior in oil and gas tract auctions in Brazil, where the main winner was Petrobras, a national company which was until recently a monopolist. We test predictions based on the theory of common-value, first-price, sealed-bid auctions with asymmetric information. The evidence indicates that Petrobras was better informed about the tracts’ values than other bidders. We show that they bid less frequently than Petrobras for tracts being reoffered after receiving no bids in previous auctions and also find that this firm was constrained in the number of auctions in which it could participate.