A sex difference in mental rotation ability is well established. Among the explanations offered for this difference are that males are more frequently right hemisphere specialised for spatial processing and that they receive more spatial processing experience because of the sex stereotyping of activities involving spatial ability that exists in Western societies. Several studies have shown that males do report more spatial activity experiences on the Spatial Activity Questionnaire of Newcombe, Bandura, and Taylor (1983) and that such experience does correlate with performances on tests of mental rotation. However, two prior lateralised tachistoscopic studies failed to show right hemisphere superiority for spatial task performance or that it was positively associated with better performances on the Vandenberg Mental Rotation Test. The tachistoscopic tasks used in these studies can be criticised on methodological grounds. The present study employed a new tachistoscopic task that showed that males were significantly more right hemisphere specialised (left visual-field superior) for the tachistoscopic task than females. Magnitudes of right hemisphere specialisation on the tachistoscopic task were positively and significantly related to mental rotation ability. Spatial activity experiences were also more common in males. Covariance and partial correlation analyses indicated that the sex difference in spatial ability was primarily due to sex differences in right hemisphere specialisation for mental rotation, while sex differences in spatial activity experiences were only secondarily involved.