Olfactory neurons expressing a given odorant receptor project with precision to 2 of the 1800 glomeruli within the olfactory bulb to create a topographic map of odor quality. We demonstrate that deletions or nonsense mutations in the P2 odorant receptor gene cause the axons of these cells to wander rather than converge on a specific glomerulus. Receptor substitution experiments that replace the P2 gene with the coding region of the P3 gene result in the projection of P3-->P2 axons to a glomerulus touching the wild-type P3 glomerulus. These data, along with additional receptor substitutions, indicate that the odorant receptor plays an instructive role in the establishment of the topographic map.