The heat shock loci of Drosophila melanogaster chromosome subdivisions 87A and 87C have been studied by using cloned DNA. Both sites contain a number of copies of a 2,4-kilobase (kb) region homologous to mRNA for the 70,000-dalton heat shock protein. In situ hybridization to chromosomal RNA shows that transcripts of this sequence accumulate at both sites after temperature elevation. At 87C there is a 1.5-kb repeated sequence homologous to another heat shock RNA. One cloned segment includes two to three tandem copies of this sequence located 0.8 kb from the beginning of a 2.4-kb message region. RNA complementary to the 1.5-kb repeat acccumulates at 87C after temperature elevation, but does not code for any known heat shock protein. In the sibling species D. simulans, there are sequences located and transcribed at 87A and 87C that are homologous to the melanogaster 2.4-kb message sequence. The 1.5-kb repeat, however, is absent from 87C in simulans and no heat shock RNA homologous to it can be detected.