The gene-sized DNA molecule in the macronucleus (mac) of Oxytricha nova (On) encoding heat-shock protein 70 (Hsp70) was cloned and sequenced. It contains 2654 bp, including telomeres. It consists of a 394-bp A+T-rich 5' leader, a 1956-bp open reading frame (ORF) encoding a putative polypeptide of 651 amino acids (aa), and a 240-bp A+T-rich 3' trailer. The 5' leader contains two copies of the 13-bp heat-shock element (HSE) consensus sequence of other eukaryotes. On responds to heat and CdCl2 stress with a major increase in hsp70 transcripts. The presence of HSE in the nucleotide sequence of hsp70 and the stress-induced in hsp70 transcripts are the first evidence of conventional transcriptional regulation of a hypotrich gene-size DNA molecule. The ORF has large putative ATPase and polypeptide-binding domains with high aa identity to Hsp70 polypeptides of other eukaryotes.