We use new data on the ownership of mutual funds in Europe to estimate how investors respond to regulations on the disclosure of Environmental, Social and Governance (ESG) performance. We find that the introduction of ESG disclosure rules for mutual funds led to strong flows into funds categorized as green. We show that investor rebalancing takes place through an uncertainty channel where investors value the lower uncertainty, and a greenness channel where funds respond to disclosure rules by increasing their greenness to attract flows. We find empirical support for both channels: green funds for which investors had little information before the regulation experience the strongest flows, and green funds that had a low ESG rating before the regulation decrease their emissions most under the new rules.