We implemented a survey experiment in the field during the (mixed gender) electoral race for mayor in Milan (2011). A sample of eligible voters was randomly divided into three groups. Two were exposed to either a positive or a negative campaign by one of the opponents. The third---control---group received no electoral information. The campaigns were administered online and consisted of a bundle of advertising tools (videos, texts, slogans). Stark gender differences emerge. Females vote more for the opponent and less for the incumbent when they are exposed to the opponent's positive campaign. Exactly the opposite occurs for males. These gender differences cannot be accounted for by gender identification with the candidate, ideology, or other observable attributes of the voters.