To investigate developmental differences in timesharing performance, 60 boys, 20 in each of three age groups (7-, 10- and 13-year-olds), performed an auditory matching task and a tracking task alone and concurrently, the latter under two sets of instructions. Decrements produced by concurrent performance were compared for the three age groups. When the timeshared tasks were presented as equally important, timesharing produced significantly greater proportional decrements in the tracking performance of the younger children. In all age groups, tracking task decrements were directly related to matching task difficulty. Subsequently, the children were instructed that one or the other of the tasks was more important and that they were to improve their performance on that task. All three age groups showed a significant improvement on the task emphasized by instructions. The relationship of the results to two models of information processing is discussed. (Author/BRT)