Forty-two male-female pairs, matched for ability, played two chess games via the Internet. When players were unaware of the sex of opponent (control condition), females played approximately as well as males. When the gender stereotype was activated (experimental condition), women showed a drastic performance drop, but only when they were aware that they were playing against a male opponent. When they (falsely) believed to be playing against a woman, they performed as well as their male opponents.