We study gender differences in relation to performance and sabotage in competitions. While we find no systematic gender differences in performance in the real effort task, we observe a strong gender gap in sabotage choices in our experiment. This gap is rooted in the uncertainty about the opponent's sabotage: in the absence of information about the opponent's sabotage choice, males expect to suffer from sabotage to a higher degree than females and choose higher sabotage levels themselves. If beliefs are exogenously aligned by implementing sabotage via strategy method, the gender gap in sabotage choices disappears. Moreover, providing a noisy signal about the sabotage level from which subjects might suffer leads to an endogenous alignment of beliefs and eliminates the gender gap in sabotage.