This paper presents new evidence of labor supply behavior consistent with workers having reference points in daily income. Moreover, consistent with a model of gain-loss utility, I find that workers with flexible labor supply exhibit increasing marginal utility in income over the loss region and diminishing marginal utility in income over the gain region. The setting is taxi drivers, a group which faces a substantial degree of uncertainty in their hourly earnings but control over when to stop working within a shift. Previous papers studying taxi drivers have found mixed evidence for reference dependent behavior. Using a driver's propensity to stop working within a shift, I show not only the existence of a reference point in a large proportion of drivers, but I am also able to estimate what the reference point is without explicit functional form or ex ante assumptions about existence. Furthermore, I show that drivers are less likely to stop working the closer they are to their reference point, indicating their preferences for daily income have a Prospect theoretic S-shape.