We test the relation between ambiguity aversion and five household portfolio choice puzzles: non-participation, low allocations to equity, home-bias, own-company stock ownership, and portfolio under-diversification. In a representative U.S. household survey, we measure ambiguity aversion using custom-designed questions based on Ellsberg urns. As theory predicts, ambiguity aversion is negatively associated with stock market participation, the fraction of financial assets in stocks, and foreign stock ownership, but positively related to own-company stock ownership. Conditional on stock ownership, ambiguity aversion is related to portfolio under-diversification, and during the financial crisis, ambiguity-averse respondents were more likely to sell stocks.