Three cloned segments of Drosophila melanogaster DNA have been isolated that derive from the major heat shock site at 87C in chromosome 3. Each of these segments contains sequences homologous to a class of polysomal poly(A) + RNAs whose synthesis is induced by heat shock of cultured cells. A combination of R loop, heteroduplex and restriction fragment maps of these segments reveals that their RNA-homologous sequences are arranged in tandemly repeated units, each unit consisting of an α element (0.49 kb) joined to a β element (1.10 kb). The polysomal RNAs homologous to these αβ units (1.59 kb) are distributed into three size classes exhibiting approximate lengths of 1, 2 and 3 kb. R loop mapping demonstrated that the sequence of the 2 kb RNA is αβα, indicating that it, and presumably the 3 kb RNA, derive from transcripts covering more than one repeated unit. One of the cloned segments contains a variant repeat unit, αγ, located between two αβ units. This unit has the same α element, but the β element has been replaced by a nonhomologous γ element (0.87 kb). Analysis of the total D. melanogaster DNA indicates that the 87C locus contains at least 21 tandemly repeated units, twelve of which were identified as αβ units and six as αγ units. The 21 or more units are distributed among at least three different tandem arrays separated by different spacer regions, one of which is within a cloned segment. Sequences in the γ element, but not those in the α or β elements, are also found at 87A, which contains another heat shock site. The possible roles of the αβ-type RNAs and of the γ sequences are discussed in the light of these results.