DnaK, the sole Escherichia coli member of the highly conserved 70-kDa heat shock protein (HSP70) family of proteins, autophosphorylates when incubated with ATP in vitro. We show that threonine-199 is the amino acid that becomes phosphorylated and we demonstrate that threonine-199 is critical for the ATPase activity of DnaK. We also report that both the ATPase and autophosphorylating activities of DnaK increase very strongly over the range of temperatures that is physiologically relevant for E. coli growth. The temperature dependence of either or both of these activities could be of significance with respect to the postulated role of DnaK as a molecular chaperone in helping cells ameliorate the deleterious consequences of elevated temperature. Furthermore, we postulate that DnaK plays a key role in regulation of the heat shock response by serving as a cellular thermometer that directly senses the environmental temperature.