Abstract: How can the formation of beneficial, lasting habits be promoted? Previous research suggests that persistent habits often involve regular, cue-triggered routines. We conducted a field experiment with 2,508 employees of a Fortune 500 company to test whether incentives for exercise routines—paying participants each time they visit a company gym within a daily two-hour window— lead to more persistent exercise behavior than flexible exercise incentives—paying participants each time they visit a company gym, regardless of the time of day. We find that an incremental gym visit in the daily two-hour window, compared to an incremental gym visit outside the window, was actually less likely to generate gym visits during the weeks after incentives were removed. Thus, while routines may be a common and important component of many lasting habits, encouraging overly rigid routines can undermine habit formation.