Effects of abacus training on cerebral functioning was investigated using a timesharing method. Three groups of children (abacus experts, moderates, and beginners), 10 to 11 years old, were asked to perform simultaneously both a key-tapping and a cognitive task. Two types of cognitive tasks were assigned to each child ; one was to recite repeatedly a series of animals' names as fast as possible and the other was to sum a series of two figures in mental calculation. The former stimuli were presented on a VDT of a personal computer and the latter were administered through headphones. Interference effects of each of the tasks upon key-tapping were measured. The results showed that (1) in the mental calculation condition key-tapping tended to be more interfered with the left than the right hand for both the experts and moderates, whereas interference was greater with respect to the right rather than the left hand for the beginners, and (2) in the verbal condition key-tapping was greatly interfered with the right hand for all three groups. These results suggested that abacus training invited the right hemisphere contribution in mental calculation.