Although extrinsic and intrinsic motivation likely jointly explain the effort of many agents engaged in public service delivery, canonical models of incentives in firms focus on the former. In the context of a rural health worker program in India, I develop and test a novel mobile phone app designed to increase agents’ intrinsic returns to effort. At one year of follow-up, the self-tracking app leads to a 24% increase in performance as measured by the main job task (home visits). Moreover, the app is most effective when it leverages pre-existing intrinsic motivation: it produces a 41% increase in performance in the top tercile of intrinsically motivated workers, but no improvement in the bottom tercile. This treatment effect persists over time for the most intrinsically motivated workers, whereas early improvements decay among the least motivated workers. Supplementary evidence suggests that the treatment effect on performance is mediated primarily by making effort more intrinsically rewarding, and not by other mechanisms such as the provision of implicit extrinsic incentives. Despite these effects on worker performance, I find no effect on health outcomes.