Mentally demanding tasks feel effortful and are usually avoided. Furthermore, prolonged cognitive engagement leads to mental fatigue, consisting of subjective feeling of exhaustion and decline in performance. Despite the intuitive characterization of fatigue as an increase in subjective effort perception, the effect of fatigue on effort cost has never been tested experimentally. To this end, sixty participants in 2 separate experiments underwent a forced-choice working memory task following either a fatigue-inducing (i.e. cognitive task involving working memory, conflict and switch costs) or a control manipulation. We measured fatigue in terms of subjective feeling and performance decrement and assessed effort in terms of subjective perception and task avoidance. Subjects exhibited only weak avoidance of the working memory task, with stronger influence of reward than task difficulty on their decisions. In addition, we found that task avoidance did not systematically change following the fatigue manipulation but that variations in task avoidance correlated with fatigue-induced performance decline. The other measures of fatigue and effort were unrelated to each other. Our findings suggest that subjective fatigue may develop independently of task avoidance and suggest an "anticipatory regulation" model in which fatigue urges subjects to stop in anticipation of possible, future adverse consequences.