Change in task persistence was assessed in two annual assessments using teachers', testers', and observers' ratings. Participants included 79 monozygotic and 116 same-sex dizygotic twin pairs who were in Kindergarten or 1st grade (4.3 to 7.9 years old) at the initial assessment. Task persistence was widely distributed and higher among older children and girls. Overall, there was modest growth in persistence over time, and moderate stability of individual differences. Most of the stability was accounted for by genetic influences, whereas most of the change was accounted for by nonshared environment, including an association with observed differential maternal warm supportive behavior.