Sexual and interspecific differences in the size of passerine bird song repertoires are related to differences in the size of song-control regions (SCR) within the brain. Most species of Thryothorus wrens (family Certhiidae) are known to duet, and, in both sexes, song repertoire sizes are related to the size of the SCR. However, one member of this genus, the Carolina wren T. ludovicianus, is very sexually dimorphic in its singing behavior: Males develop large song repertoires, whereas females do not sing. In this study, Nissl staining was used to investigate whether the marked gender difference in the behavior of this species is related to sexual dimorphism of the SCR. Carolina wren males, as predicted, possess the largest premotor song nuclei within the genus; these nuclei could not be identified within Nissl-stained female tissue. The cellular bases for gender differences in SCR morphology also were examined: Males and females differed strongly in the size and density of neurons making up the regions in which SCRs exist in the male forebrain. Interspecific comparison provided no evidence for a decoupling of behavioral and neural evolution within this clade. Male Carolina wrens possess the largest song repertoires and SCRs within the genus, whereas females of this species represent the opposite behavioral and neural extremes of this songbird group. These results are consistent with the hypothesis that the size of the passerine song repertoire is limited by the amount of neural tissue devoted to singing.