Gender differences in hemispheric asymmetry for language functioning have been reported in the neuropsychological literature. Despite numerous reports of anatomic asymmetries in corresponding cortical regions, the possibility of gender dimorphism in the putative neuroanatomical substrate of language has not been systematically examined in vivo. We assessed asymmetry of the planum temporale (PT), a supratemporal region of auditory association cortex, in 12 normal, right-handed females and 12 age-matched right-handed males with the aid of MRI surface-rendering morphometry. Bilateral areas were also assessed for Heschl’s gyrus (HG), a supratemporal region of primary auditory cortex where no asymmetry was anticipated. We found a significant interaction between gender and hemisphere for the PT, with males having significantly larger left versus right PTs. Left-right differences in PT area were not significant among females (10 of 12 males showed leftward lateralization of the PT, vs 5 of 12 females). No main effect of gender was found for total (left + right) PT area, and no asymmetries or gender effects were detected for HG. This finding of gender dimorphism in PT area is consistent with evidence for reduced asymmetry among females in the lateralization of language functions attributable to the supratemporal cortex. The implications for theories about interactions between sex hormones and the development of brain asymmetries are discussed.