<p>This paper examines behavior in contests where the prize value is ambiguous. We develop a theoretical model of bidding in a Tullock contest with an ambiguous prize where contestants account for the ambiguity attitude of their rival. Ambiguity affects optimal behavior via two countervailing channels - a direct effect arising from contestants’ ambiguity about the value of the prize and an indirect effect corresponding to the effect of ambiguity on the opponent’s behavior. Using a controlled laboratory experiment, we elicit individual risk and ambiguity attitudes and compare predicted and observed behavior in contests with an ambiguous prize, a risky prize and certain prizes. A comparison between contests with ambiguous and risky prizes, shows that participants invest significantly less under ambiguity. Additionally, we decompose the effect of changing from a certain prize to an ambiguous prize into two components - the first is the effect of introducing risk and the second is the effect of introducing ambiguity. Empirically, we find that both effects are significant, but work in opposite directions.</p>