Parameters of thermal death were determined in 10 strains of yeast species whose maximum temperatures for growth (T max) ranged from 22 to 49°C. Arrhenius plots of the specific thermal death rates (k d) formed a positional sequence at the level of the experimental points that corrresponded in all but one case to the sequence of the respective T max values. Extrapolated k d values at higher or lower temperatures no longer formed this sequence. The correlation of the temperature functions with T max could be characterized in terms of a new activation parameter, for which the name thermal death activation constant is introduced. It has the following form: T.D.A. $$constant = \frac{{\Delta {\rm H} \ne }}{{T_{\max + n} }}$$ − ΔS≠ where ΔH≠ and ΔS≠ are respectively the apparent heat and entropy of activation of thermal death and n is the number of degrees above T max (expressed in °K) at which the T.D.A. constant exists. Seven mesophilic yeasts had a T.D.A. constant between 72 and 79 calxmol-1 degree-1 at n values between 1 and 4°. This suggested that the destructive process that limits k d in these strains is of the same species as one that contributes to the establishment of T max. Two psychrophilic yeasts apparently had a similar T.D.A. constant but at a high n value (about 12.5°C) which suggested that in these strains T max is governed by a destructive process unrelated to the one that underlies thermal death. The strain of the nearly thermophilic Hansenula angusta (T max 49°C) did not fit in either group. The significance of the T.D.A. constant is discussed and expressions for ΔH≠ and ΔS≠ in terms of bond activation parameters are proposed.