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