Fibroblast growth factor-2 (FGF2) has been postulated to be a key regulator involved in the proliferation, differentiation, and regeneration of sensory hair cells. Here we have addressed the potential functions of FGF2 during the formation and regeneration of the auditory epithelium in chicken and mice. By using viral gene transfer, based on herpes simplex type 1 virus (HSV-1), we show that ectopically applied FGF2 drastically increases the number of cells expressing early hair cell markers during embryonic development in avians. Intriguingly, FGF2 does not stimulate cell division during this process. These data suggest that FGF2 plays a role during differentiation of sensory hair cells in avians. To address the potential functions of FGF2 during murine inner ear development, we analyzed FGF2 mouse mutants. Mice lacking FGF2 showed normal formation of the inner ear, and no abnormalities were observed at the adult stage. Moreover, FGF2 mouse mutants showed similar hearing thresholds compared with those observed in control mice before and after noise damage. Therefore, endogenous FGF2 appears not to be essential for the development or functional maintenance of the auditory organ in mammals. In light of these results, the differential roles of FGF2 in the vertebrate inner ear are discussed with respect to its previously postulated functions.