Do women’s motives for joining a political party depend on the implementation of a gender quota for party offices and electoral lists? Based on multiple waves of the German Party Membership Studies, we address this issue using survey data for about 5,000 female party members. We find that women’s probability of joining a party because of their political ambition, i.e. attaining a public or party office, slightly improves with the quota size. But more importantly, there is a robust, statistically significant interaction between the quota targets and the percentage of female party members. Women are most likely to report instrumental reasons for joining when there is a large gender gap for the party members and a quota requiring a high percentage of female office holders. These results strongly imply that women, at least roughly, calculate the intensity of the intra-party competition for political offices when deciding to join a party.