Phenotypic analysis of the daf-11 and daf-21 mutants of Caenorhabditis elegans suggests that they have defects in components shared by processes analogous to vertebrate taste and olfaction. daf-11 and daf-21 mutations were previously shown to cause inappropriate response to the dauer-inducing pheromone. By mutational analysis and by disabling specific chemosensory sensilla with a laser, we show that neurons in the amphid sensilla are required for this pheromone response. Using behavioral assays, we find that daf-11 and daf-21 mutants are not defective in avoidance of certain non-volatile repellents, but are defective in taxis to non-volatile attractants. In addition, both mutants are defective in taxis to volatile attractants detected primarily by the amphid neuron AWC, but respond normally to volatile attractants detected primarily by AWA. We propose that daf-11 and daf-21 mediate sensory transduction for both volatile and non-volatile compounds in specific amphid neurons.