SSA4 is the only one of five heat-inducible HSP70 genes in Saccharomyces cerevisiae whose expression is restricted to conditions of stress. Comparison of the nucleotide sequences of the SSA4 gene with other HSP70 genes indicates that it diverged from its most closely related yeast homologues hundreds of millions of years ago. However, a high degree of identity has been maintained between Ssa4p and other yeast 70-kDa heat-shock proteins at the amino acid level suggesting, in light of its distinct pattern of regulation, that it performs an important function. A 44-base pair region of the SSA4 promoter containing an extended match to the conserved eukaryotic heat-shock element (HSE) is necessary and sufficient to mediate heat-inducible regulation. HSESSA4 is capable of promoting only a low level of transcription under nonstress conditions. We present evidence in support of a revised definition of the functional HSE in S. cerevisiae, similar to the recently proposed modular Drosophila HSE. Elevated expression of several heat-shock proteins in an ssa1ssa2 double-mutant strain has previously been reported. The SSA4 promoter is activated in this strain. The increase in expression of SSA4 caused by deletion of these closely related genes is mediated via the same upstream activating sequences that activate transcription in response to heat shock. Activation of HSE-mediated transcription by disruption of constitutively expressed HSP70 genes supports an autoregulatory model of control of the heat-shock response.