Morphological asymmetry of the frontal operculum and temporal planum becomes measurable at the 29th week of gestation. There is evidence of subsequent differential development of the planum in favor of the left, with the left planum larger than the right. While both the frontal operculum and left planum were always present, the right planum ranged in size from absent (10%)to larger than the left (about 10%). Females predominated (P less than .05) in the latter group. The findings suggest that (1) a higher percentage of persons may have right-sided cerebral representation for speech than has been assumed previously; (2) a predetermined morphological asymmetry contributes to establishing the ultimate pattern of cerebral speech representation following an early insult to a predisposed hemisphere; and (3) it is necessary to scrutinize clinical material for the differential organization of hemispheric development between sexes.