The correct identity and functional capacity of transplanted dopamine (DA) neurons derived in vitro from embryonic stem (ES) cells is a critical factor for the development of an ES cell-based replacement therapy for Parkinson's disease. We transplanted primate Cyno-1 ES cells differentiated in vitro for 4 (progenitor ES cells) or 6 (differentiated ES cells) weeks, or control fetal primate cells into the striatum of hemi-parkinsonian rats. Partial behavioral recovery in amphetamine-induced rotation was correlated with the number of ES-derived tyrosine hydroxylase-positive (TH +) neurons in the grafts (r = 0.5, P < 0.05). Post mortem analysis of ES-derived grafts revealed TH + neurons with mature morphology, similar to fetal DA neurons, and expression of midbrain transcription factors, such as Engrailed (En) and Nurr-1. While the total number of TH + neurons was not different between the two groups, TH/En co-expression was significantly higher (> 90%) in grafts from differentiated ES cells than in grafts derived from progenitor cells (< 50%), reflecting a more heterogeneous cellular composition. Within the grafts there was an overlap between ES-derived TH + axonal arbors and clusters of primate ES-derived striatal neurons expressing brain factor 1 (Bf-1, Foxg1) and DA and cAMP-regulated phosphoprotein (DARPP-32). Such overlap was never observed for other regional transcription factors that define neighboring forebrain domains in the developing brain, such as Nkx2.1 (medial ganglionic eminence), Nkx2.2 (pallidal and diencephalic progenitors) or Pax6 (dorsal telencephalic progenitors). Despite the heterogeneity of ES-derived graft cell composition, these results demonstrate normal phenotypic specification, conserved natural axonal target selectivity and functionality of DA neurons derived from primate ES cells.