A young acallosal man was intensively tested in a standard simple reaction time (RT) paradigm using briefly-presented lateralized spots for light. In Experiment 1, findings on previous acallosal patients of a large disadvantage for crossed (e.g. right hemifield-left hand) as against uncrossed (e.g. left hemifield-left hand) RTs were replicated. This crossed-uncrossed difference (CUD), as in previous work, turned out to be smaller in a bimanual response task than in the conventional unimanual task. Experiment 2 was a factorial study of unimanual RTs in which (a) stimulus intensity and (b) spatial S-R compatibility, were varied. As in a previously tested patient, decreased intensity resulted in a greatly increased CUD. S-R compatibility on the other hand had no effect on CUD. The results are interpreted as favouring a role for visual commissural neurones in the acallosal CUD, and as evidence against a spatial compatibility hypothesis.