Androgens play a central role in the regulation of male sexual differentiation and behavior in many vertebrates, including zebrafish. Their signaling is mediated by activation of the androgen receptor. A single androgen receptor (ar) gene was recently identified in zebrafish, which encodes a protein that binds androgens in vitro. However, the tissue-specific expression pattern of this receptor in vivo has not been described. Using whole-mount RNA in situ hybridization, we characterized expression of the ar gene in developing zebrafish and in the adult brain. In embryos, transcripts were found in the presumptive pronephros and in olfactory placodes. By 3–5 days postfertilization, ar transcripts were also detected in the pineal organ anlage and the retina. In the adult brain, ar was expressed in discrete regions of the telencephalon, in the preoptic area, and throughout the periventricular hypothalamus, regions previously implicated in the regulation of sexually dimorphic behaviors in mammals. Developmental Dynamics 237:2987–2995, 2008. © 2008 Wiley-Liss, Inc.