This paper examines how mutual funds’ portfolio holdings respond to environmental regulations. Using county-level ozone nonattainment designations induced by discrete policy changes in the National Ambient Air Quality Standards as a source of exogenous variation in local regulatory stringency, we find that funds underweight (overweight) those polluting stocks whose cash flows covary negatively (positively) with the regulatory shock. Our results are consistent with active portfolio rebalancing in response to expected changes in firm fundamentals due to negative cash flow shocks stemming from the costs of nonattainment regulation. Further analyses in the post-nonattainment period show that stocks with high exposure to nonattainment designations exhibit worse operating performance and increased regulatory compliance costs. The most underweighted of such firms also exhibit worse abnormal stock return performance. Funds that reduce their portfolio exposure to nonattainment designations see an improvement in their investment performance.