Transcription at two heat shock loci in Drosophila melanogaster, in subdivisions 87A and 57C, was investigated by hybridization in situ with 3H-labeled messenger, nuclear RNA and whole cell RNA from cells cultured at elevated temperature. What appears to be the same 9 x 10(5) dalton heat shock message hybridizes at both sites. At 87a, little additional hybridization is obtained with nuclear or whole cell RNA. In contrast, at 87C the saturation level of hybridization by nuclear and whole cell RNA is much higher than that obtained with the message alone. This evidence for extensive hybridization at 87C but not at 87A by RNA distinct from the message is confirmed by the finding that excess nonradioactive message competes away most of the hybridization by 3H-labeled nuclear and whole cell RNA at the latter locus but not at the former. The noncompetable RNA migrates on an electrophoretic gel as a heterogeneous population of molecules, extending to sizes both larger and smaller than the message. These and other observations lead to the conclusion that a 87A transcription includes little more than sequences complementary to the 9 x 10(5) dalton message, while at 87C, there are sequences complementary to the same message and extensive additional sequences complementary to other species of RNA.