The spatial organization of projections from olfactory receptor neurons to the main olfactory bulb (MOB) was studied in hamsters by using fluorescent stilbene isothiocyanates as retrograde tracers. Injections confined to small sectors of the MOB produce labeling of receptor neurons that is more restricted circumferentially (i.e., with respect to the medial-lateral and dorsal-ventral axes) than longitudinally (i.e., with respect to the rostral-caudal axis) along the mucosal sheet. This restricted labeling is also discontinuous, giving an initial impression that the peripheral input is only crudely organized with respect to the medial-lateral and dorsal-ventral axes of the nasal cavity. However, from analyses of serial sections, it is apparent that each set of mucosal segments shares convergent projections to a circumferential quadrant of the MOB with other segments that are positioned around a common domain of the nasal cavity airspace. The primary afferent projections to the MOB, thus, are organized rhinotopically (i.e., with respect to the three-dimensional position of receptor neurons in olfactory space) rather than mucosotopically.