The dauer larva is a specialized third-larval stage of Caenorhabditis elegans that is long-lived and resistant to environmental insult. The dauer larva is formed in response to a high external concentration of a constitutively secreted pheromone. Response to the dauer-inducing pheromone of C. elegans is a promising genetic model for metazoan chemosensory transduction. More than 20 genes have been identified that are required for normal pheromone response. The functions of these genes include production of the pheromone, exposure of sensory neuron endings to the environment, structural and functional integrity of those sensory endings, and the capacity of sensory neurons to make appropriate output. Genetic evidence suggests that two partially redundant sensory pathways act in concert to control dauer formation. At least two classes of chemosensory neurons, ADF and ASI, are implicated in the pheromone response. On the basis of on these findings, a speculative model for the pheromone response is proposed. In this model, the neurons ADF and ASI are pheromone sensors that repress dauer formation in the absence of pheromone and derepress dauer formation in response to pheromone. It is currently unclear whether or not the two genetically defined sensory pathways both act in ADF and ASI.