ABSTRACT Out of 25,000 EMS-treated third chromosomes examined, ten dominant temperature-sensitive (DTS) lethal mutations which are lethal when heterozygous at 29°C but survive at 22°C were recovered. Seven of the eight mutations mapped were tested for complementation; these mutants probably define eight loci. Only DTS-2 survived in homozygous condition at 22°C; homozygous DTS-2 females expressed a maternal effect on embryonic viability. Two of the mutant-bearing chromosomes, DTS-1 and DTS-6, exhibited dominant phenotypes similar to those associated with Minutes. Each of the seven mutants examined exhibited a characteristic phenotype with respect to the time of death at 29°C and the temperature-sensitive period during development. Only DTS-4 exhibited dominant lethality in triploid females.