Illegitimate tasks, a recently introduced occupational stressor, are tasks that violate norms about what an employee can reasonably be expected to do. Because they are considered a threat to one’s professional identity, we expected that the daily experience of illegitimate tasks would be linked to a drop in self-esteem and to impaired well-being. We report results of two daily diary studies, one in which 57 Swiss employees were assessed twice/day and one in which 90 Americans were assessed three times/day. Both studies showed that illegitimate tasks were associated with lowered state self-esteem. Study 1 demonstrated that high trait self-esteem mitigated that relationship. Study 2 showed that illegitimate tasks were associated with not only lowered state self-esteem but also lower job satisfaction and higher anger and depressed mood. depressive mood, but not anger or job satisfaction remained elevated until the following morning.