As a result of a mandatory board gender quota regulation, the percentage of female directors in Norway increased from around 5% in 2001 to over 40% in 2007, while it remained stable in neighbouring Denmark. Taking advantage of this unique research setting, this study implements a difference-in-differences approach to investigate the effects of the gender composition of the board of directors on corporate tax aggressiveness. Results indicate that the likelihood of corporate tax aggressive strategies increased in Norway after the appointment of many female directors, compared to the situation in Denmark. This finding is robust to a battery of sensitivity analyses and, in particular, to how corporate tax aggressiveness is measured. We interpret this result as caused by the way in which the incorporation of women to the boards was achieved, that is, through a mandatory board gender quota regulation. Possible implications of the findings are discussed.