We conducted a field experiment to test whether savings constraints prevent the self-employed from increasing the size of their businesses. We opened interest-free savings accounts in a village bank in rural Kenya for a randomly selected sample of poor daily income earners. Despite the fact that the bank charged substantial withdrawal fees, take-up and usage was high among women and the savings accounts had substantial, positive impacts on their productive investment levels and expenditures. These results imply that a substantial fraction of daily income earners face important savings constraints and have a demand for formal saving devices (even for those that offer negative de facto interest rates).