We study experimentally the selection into first-price sealed-bid auctions for a risky or an ambiguous prospect. Most subjects chose to submit a bid for the risky prospect, leading to thinner markets for the ambiguous prospect. Transaction prices for both prospects were equal although subjects expected the ambiguous markets to be smaller. Evidence of a positive correlation between risk and ambiguity aversion suggests that the ambiguous markets were populated by relatively risk tolerant bidders. A control experiment with selection in a simple choice task shows that subjects correctly anticipate the effects of selection on market size and risk attitudes. (JEL C91, D44, D81)