The retinoic acid receptor RARβ is highly expressed in the striatum of the ventral telencephalon. We studied the expression pattern of different RARβ isoforms in the developing mouse striatum by in situ hybridization. We found a differential ontogeny of RARβ2 and RARβ1/3 in embryonic day (E) 13.5 lateral ganglionic eminence (striatal primordium). RARβ2 mRNA was detected primarily in the rostral and ventromedial domains, whereas RARβ1/3 mRNAs were enriched in the caudal and dorsolateral domains. Notably, by E16.5, a prominent decreasing gradient of RARβ2 mRNA was present in the developing striatum along the rostrocaudal axis, i.e., RARβ2 was expressed at higher levels in the rostral than the caudal striatum. No such gradient was found for RARβ1/3 and RARβ3 mRNAs. The rostrocaudal RARβ2 gradient gradually disappeared postnatally and was absent in the adult striatum. The differential expression pattern of RARβ isoforms in the developing striatum may provide an anatomical basis for differential gene regulation by RARβ signaling. Developmental Dynamics 233:584–594, 2005. © 2005 Wiley-Liss, Inc.