In speedboat racing in Japan, men and women compete under the same conditions and are randomly assigned to mixed-sex or single-sex groups for each race. We use a sample of over 140,000 individual-level records to examine how male-dominated circumstances affect women's racing performance. Our fixed-effects estimates reveal that women's race time is slower in mixed-sex than all-women races, whereas men's race time is faster in mixed-sex than men-only races. The same result is found for place in race. Moreover, in mixed-sex races, men are more aggressive, as proxied by lane changing, than women in spite of the risk of being penalized for rule infringement.