Neural stem cells (NSCs) have become promising tools for basic research and regenerative medicine. Intracerebral transplantation studies have suggested that these cells may be able to adopt neuronal phenotypes typical of their engraftment site and to establish appropriate connections in the recipient circuitries. Here, we examined the in vivo neurogenic competence of well-characterized NSC lines subjected to in vitro priming and subsequent implantation into the adult intact mouse brain. Upon implantation into the hippocampus and, less frequently, in the striatum and in the cerebral cortex, numerous green fluorescent protein (GFP)-tagged cells acquired differentiated features indistinguishable from resident neurons. Upon closer examination, however, we found that this outcome resulted from fusion of donor cells with local neuronal elements generating long-term persistent GFP(+) neuronal hybrids. This fusogenic behavior of NSCs was unexpected and also observed in coculture with E18 hippocampal immature neural cells, but not with microglia or astrocytes. Similar findings were consistently obtained with different NSC lines, mouse recipients, and donor cell-labeling methods. The frequent and cell type-specific fusion of donor NSCs with host neurons highlights a previously underestimated biological property of the nervous tissue that might prove profitable for basic and therapeutically oriented studies.