Women are underrepresented at senior levels in most companies. This article examines whether gender differences in the propensity to negotiate contribute to this pattern. Based on a behavioral experiment run in a major investment bank in the United States, I found that fewer women than men are willing to negotiate, but employees who have a propensity to negotiate are promoted on average seventeen months more quickly than those who do not. Women advance more slowly than men, which accounts for the underrepresentation of women in senior positions. I conclude that gender differences in the propensity to negotiate partially explains why women are on a “slow elevator” to the top.