The heat shock transcription factor (HSF) is a evolutionarily conserved activator of the heat shock protein and the expression of the hsp gene in eukaryotes. The temperature-sensitive hsf⁴ mutation in Drosophila melanogaster was originally considered to be a complete loss of function. We found that the HSF⁴ function is only lost temporarily after heat exposure, and is eventually recovered, even after severe heat stress, which manifests in the induction of hsp gene expression. We investigated the kinetics of the accumulation and degradation of the transcript and protein products in the hsf⁴ strain and found that the induction of hsp genes upon recovery is still weaker than in the wild-type strain. Therefore, the hsf⁴ is as hypomorphic allele. Although the HSF⁴ protein can induce the hsp genes, the response to heat shock in the hsf⁴ strain remains defective.