Ethyl methanesulfonate-treated third chromosome of Drosophila melanogaster were tested for the presence of dominant and recessive temperature-sensitive lethal mutations at 17 degrees , 22 degrees and 29 degrees C. Out of 1,176 chromosomes tested, no dominant ts lethals, 21 heat-sensitive, 22 cold-sensitive and 10 heat-cold-sensitive lethals were recovered. Heat-cold sensitivity was produced by a single mutation in all cases. Sixty-two percent of the ts lethals were fertile as homozygotes in both sexes. Surprisingly, 88% of the ts lethals mapped between st and Sb, a region straddling the centromere and estimated to comprise 12.9% of the genetic length and 55% of the physical length of chromosome 3. All but one of the heat- and cold-sensitive lethals complemented with each other at their respective restrictive temperatures.