A Caenorhabditis-specific pheromone and the food supply influence both entry into and exit from a developmentally arrested juvenile stage called the dauer larva. The pheromone increases the frequency of dauer larva formation and inhibits recovery but does not affect adult behavior such as chemotaxis and egg laying. The fatty acid--like pheromone has been partially purified and characterized by a new bioassay. If similar developmental control mechanisms are used by parasitic nematodes, such mechanisms might be exploited to develop highly selective anthelmintic agents.