Steroid hormones and receptors play a role in regulating biological events underlying brain development and sexual differentiation. Current evidence indicates that circulating sex steroid hormones are not entirely responsible for development of neural sex differences in song birds such as the zebra finch. p68, as a coactivator specific for estrogen receptor alpha (ERα) and an essential factor in early tissue development and maturation might play a role in sexual differentiation. Zebra finches have a sexually dimorphic song control nuclei in the brain, males have larger song nuclei than females, and are ideal model for investigating the mechanisms controlling sexual differentiation of the brain and behavior. Western blot analysis showed a significant sex difference at post hatch day 10 (P10). Immunohistochemistry showed localization of p68 immunoreactive cells in the ZF brain including nuclei that compose the avian song system. p68 is probably developmentally regulated and may be modulated by endogenous estrogen and estrogen receptors suggesting a role for p68 in sexual differentiation. INDEX WORDS: p68, coactivator, RNA helicase, steroid receptor, song control nuclei, zebra finch (ZF)