Shocks to the supply of long-term bonds affect the duration of corporate investment. Using large and plausibly exogenous shocks to the maturity structure of US government debt, I find that a higher supply of long-term bonds increases firms’ financing costs at long horizons leading to a crowding-out of long-duration investment. I show that this crowding out occurs through reallocations of capital away from long-duration investment towards short-duration investment, not only across industries but also within industries across firms and within firms across divisions. I show that these changes to the average duration of investment map into changes to the average maturity of corporate debt. These results identify important real effects of policies which affect the net supply of long-term bonds, such as quantitative easing by central banks.