Heat‐shock genes coding for heat‐shock protein 70 (HSP70) in Drosophila melanogaster were subcloned into an SV40/plasmid recombinant capable of replication in permissive monkey COS cells. Following transfection of COS cells, no significant amount of Drosophila hsp70 RNA was detected at 37 degrees C. In contrast, a heat‐shock at 43 degrees C or arsenite poisoning at 37 degrees C induced the massive production of Drosophila hsp70 RNA of correct size and faithful 5′ ends. After heat‐shock, the efficiency of hsp70 transcription in COS cells containing 2‐4 X 10(4) gene copies was found to be 15‐30% of that measured in Drosophila, on a per gene basis. By testing a series of 5′ deletion mutants in this inducible transcription assay it was found that a sequence less than 70 bp long, directly upstream of the hsp70 gene, was essential for the heat or arsenite induction of transcription.