Following prolonged exposure to an odorant, C. elegans exhibits a diminished response to the odorant for several hours. This olfactory adaptation is odorant selective; animals can adapt independently to different odorants sensed by a single pair of olfactory neurons, the AWC neurons. The mechanism of olfactory adaptation is genetically complex, with different genes required for adaptation to different odorants. Animals mutant for the gene adp-1 fail to adapt to a subset of AWC-sensed odorants; adp-1 affects a calcium-dependent process required for adaptation. Mutations in another gene, osm-9, affect adaptation to a different but overlapping subset of AWC-sensed odorants. Mutations in adp-1 and osm-9 do not diminish the ability of unadapted animals to respond to odorants, indicating that odorant sensation and odorant adaptation are distinct processes.