I estimate a dynamic investment model for mutual managers to study the cross-sectional distribution of ability, incentives, and risk preferences. The manager's compensation depends on the size of the fund, which fluctuates due to fund returns and due to fund flows that respond to the fund's relative performance. The model provides an economic interpretation of time-varying coefficients in performance regressions in terms of the structural parameters. I document that the estimates of fund alphas are precise and virtually unbiased. I find substantial heterogeneity in ability, risk preferences, and pay-for-performance sensitivities that relates to observable fund characteristics.