Induction of HSP70 heat shock genes by light has been demonstrated in Chlamydomonas. Our aim was to establish whether this induction by light is mediated by the heat stress sensing pathway or by an independent signal chain. Inhibitors of cytoplasmic protein synthesis revealed an initial difference. Cycloheximide and other inhibitors of protein synthesis prevented HSP70A induction upon illumination but not during heat stress. Analysis of HSP70A induction in cells that had differentiated into gametes revealed a second difference. While heat shock resulted in elevated HSP70A mRNA levels, light was no longer able to serve as an inducer in gametes. To identify the regulatory sequences that mediate the response of the HSP70A gene to either heat stress or light we introduced a series of progressive 5' truncations into its promoter sequence. Analyses of the levels of mRNA transcribed from these deletion constructs showed that in most of them the responses to heat shock and light were similar, suggesting that light induction is mediated by a light-activated heat shock factor. However, we show that the HSP70A promoter also contains cis-acting sequences involved in light induction that do not participate in induction by heat stress. Together, these results provide evidence for a regulation of HSP70A gene expression by light through a heat shock-independent signal pathway.