Alpha wave (8-12Hz) activity was monitored over the four lobes of the left and right cerebral hemispheres while 12 male and 12 female participants mentally manipulated circles and arcs to determine size matches. Four task conditions were tested, each designed to tap into the visual encoding, image generation, mental rotation, and comparison/decision subcomponents comprising this task. Males were more accurate than females in making such matches, and in all conditions they exhibited a significant alpha power reduction asymmetry, with the right frontal lobe being more active than the left frontal lobe. Females showed a pattern of activation that was bilaterally distributed, primarily to the frontal and temporal regions. These findings suggest a difference in the functional organisation of the male and female brain which may contribute to the reported sex difference in the performance of some spatial tasks.