We study entry and bidding patterns in sealed bid and open auctions. Using data from the U.S. Forest Service timber auctions, we document a set of systematic effects: sealed bid auctions attract more small bidders, shift the allocation toward these bidders, and can also generate higher revenue. A private value auction model with endogenous participation can account for these qualitative effects of auction format. We estimate the model’s parameters and show that it can explain the quantitative effects as well. We then use the model to assess bidder competitiveness, which has important consequences for auction design.