Soil dwelling nematodes encounter many types of volatile and water-soluble molecules in their environment. For free-living nematodes like Caenorhabdiris clegnns, successful foraging depends on the ability to detect a gradient in one odorant while ignoring extraneous odoun. The infectious stages of plant and animal parasitic nematodes also rely on chemoreception as their primary host finding cue. Using a combination of genetic, molecular and bioinformatic approaches chemoreceptor genes have been identified in C. clegans. These C. elegans chemoreceptor genes encode seven-transmembrane G-protein coupled receptors (GPCR) and comprise the largest gene family in this nematode. GPCR are also involved in olfactory signal transduction across a broad spectrum of animals including insects, crustaceans, fish and mammals, but the C. elegans (and Drosophila) chemoreceptor genes have no sequence homology to vertebrate GPCR odour receptor genes and they also differ from vertebrate odour receptor genes in their genomic structure. We review the genomic structure and diversity of odorant and chemoreceptor gene families in vertebrates and invertebrates and describe our attempts, using homology-based approaches, to isolate chemoreceptor genes in the entomopathogenic nematode Heterorhabditis bacteriophora.