Semaphorins are known to play an important role in axon guidance of vertebrate olfactory sensory neurons to their targets in specific glomeruli of the olfactory bulb (OB). However, it is not clear how semaphorin-mediated guidance contributes to a systematic hierarchy of cues that govern the organization of this system. Because of the putative role that odorant receptor molecules such as P2 could play in establishing appropriate glomerular destinations for growing olfactory axons, we have also determined the spatial organization of P2 glomeruli in semaphorin 3A (Sema3A) mutant mice. First, in the postnatal OB of control and Sema3A(-/-) mice, we analysed the trajectories of olfactory axons that express the Sema3A receptor, neuropilin-1 (npn-1) and the positions of npn-1(+) glomeruli. Sema3A at the ventral OB midline guides npn-1(+) axons to targets in the lateral and medial OB. Absence of Sema3A permits many npn-1 axons to terminate aberrantly in the rostral and ventral OB. Second, in Sema3A(-/-) mice, many P2 axons are abnormally distributed throughout the ventral OB nerve layer and converge in atypical locations compared with littermate controls where P2 axons converge on stereotypically located lateral and medial glomeruli. In addition to their radically altered spatial distribution, P2 glomeruli in Sema3A(-/-) mice are significantly smaller and more numerous than in heterozygote littermates. These data show that Sema3A is an important repulsive olfactory guidance cue that establishes restricted npn-1(+) subcompartments in the olfactory bulb. Furthermore, Sema3A plays a key role in the convergence of axons expressing the odorant receptor P2 onto their appropriate targets.