This study examines the effect of gender quotas in European Parliament (EP) elections on the legislative participation of Members of the European Parliament (MEPs), and how this effect varies by gender, party composition, ideological leaning, and policy domain. Drawing on a comprehensive dataset of 3,462 legislators and nearly 37,000 roll call votes spanning four EP terms, we apply a staggered difference-in-differences approach to isolate the causal effect of quotas implemented at the level of national party delegations. Our analysis reveals that quotas substantially increase legislative turnout among female MEPs—particularly when they remain a minority within their parties. Moreover, this positive effect extends to both left- and non-left-leaning parties. We also find that quota-driven increases in female MEPs' legislative participation are concentrated in policy areas often associated with women's interests, such as gender equality, education, public health, development, and environmental sustainability. These findings enhance our understanding of how gender balance mechanisms reshape legislative behavior in a supranational setting.