It is well established that an increase in cognitive task demands is associated with increased pupil diameter. However, the effect of increased motor task demands on pupil diameter is less clear. Previous research indicates that higher motor task complexity increases pupil diameter but suggests that higher motor task precision demands may decrease pupil diameter during task movement. The current study investigated the effect of increased motor task precision on pupil diameter using a Fitts' Law movement task to manipulate motor response precision. Increased precision demands were associated with reduced pupil diameter during the response preparation and response execution phases of the movement trials. This result has implications for the interpretation of pupil diameter as an index of workload during tasks which involve precise motor movements.