First described in 1973, the Grueneberg ganglion (GG) is an arrow-shaped neuronal structure at the anterior end of the nasal cavity. It lines both sides of the nasal septum, within the nasal vestibule, close to the opening of the naris. The functions of the GG and the pattern of projections to the brain are not known. Here, we report that neurons of the mouse GG express olfactory marker protein, which is normally expressed in mature olfactory or vomeronasal sensory neurons. The approx. 500 cells in each GG are arranged in several densely packed cell clusters. Individual cells give rise to single axons, which fasciculate to form a nerve bundle that projects caudally. The axons terminate in glomeruli of the olfactory bulb, one or two large glomeruli associated with a semicircle of up to 10 smaller, somewhat diffusely organized glomeruli that surround the most anterior part of the accessory olfactory bulb. Development of the GG starts around embryonic day 16 and appears to be completed at birth; cell numbers then undergo a minor decrease during postnatal development. The strategic location of the GG, expression of olfactory marker protein, axonal projections to glomeruli at particular locations in the olfactory bulb and early development suggest that this neuronal structure performs specific chemosensory functions at neonatal stages.