Formation of a developmentally arrested dispersal stage called the dauer larva is enhanced by a Caenorhabditis-specific pheromone and is inhibited by increasing amounts of food. Pheromone-induced dauer larva formation of three tested wild-type strains is temperature-dependent, so that an increased percentage of the population forms dauer larvae at 25 degrees C compared to lower temperatures. Dauer-defective mutants fail to respond to added pheromone, and some behavioral mutants affected in thermotaxis or egg-laying also exhibit abnormal responses. Temperature-sensitive (ts) dauer-constitutive mutants form dauer larvae at a restrictive temperature regardless of environmental stimuli. At the permissive temperature (17.5 degrees C), alleles of six out of seven dauer-constitutive genes tested overrespond to the dauer-inducing pheromone. All known mutations in daf-4 (eight alleles) and daf-7 (five alleles) produce a ts dauer-constitutive phenotype. One daf-4 and one daf-7 allele are suppressed by the amber nonsense suppressor, sup-7(st5). At least these two dauer-constitutive mutations are likely to cause production of nonfunctional rather than ts gene products. These mutations appear to indirectly result in a ts phenotype by enhancing the expression of a wild-type ts developmental process.