Changes in the environment cause both short-term and long-term changes in an animal's behavior. Here we show that specific sensory experiences cause changes in chemosensory receptor gene expression that may alter sensory perception in the nematode Caenorhabditis elegans . Three predicted chemosensory receptor genes expressed in the ASI chemosensory neurons, srd-1 , str-2 , and str-3 , are repressed by exposure to the dauer pheromone, a signal of crowding. Repression occurs at pheromone concentrations below those that induce formation of the alternative dauer larva stage, suggesting that exposure to pheromones can alter the chemosensory behaviors of non-dauer animals. In addition, ASI expression of srd-1 , but not str-2 and str-3 , is induced by sensory activity of the ASI neurons. Expression of two receptor genes is regulated by developmental entry into the dauer larva stage. srd-1 expression in ASI neurons is repressed in dauer larvae. str-2 expression in dauer animals is induced in the ASI neurons, but repressed in the AWC neurons. The ASI and AWC neurons remodel in the dauer stage, and these results suggest that their sensory specificity changes as well. We suggest that experience-dependent changes in chemosensory receptor gene expression may modify olfactory behaviors.