Summary.The spores ofBotrytis cinereawere exposed to the action of hot water at temperatures ranging from 31oC. to 50‐3oC, the proportion of spores surviving at different times determined, and the results plotted for each temperature. These give a series of approximately symmetrical sigmoid curves, all exactly alike one another except for the change in the speed of killing at different temperatures. The curves are superimposable on adjustment of the time scale, and in this respect they differ from the curves obtained with phenol, where the shape changes from an S to a J shape as the strength of phenol is increased, eventually becoming logarithmic.The shape of the general curve agrees excellently with a recognised type of frequency distribution, and the observations at all the temperatures examined fall closely on the curve of this distribution.The effect of temperature on the velocity of the reaction is unusually great, and is well expressed by the formula of Arrhenius, if the temperature is reckoned from 0oC. instead of the absolute temperature.By combination of the formula for the curve and the formula for the velocity‐temperature relationship it is possible to express completely for the spores ofBotrytisthe whole of the killing process for any temperature within the Mmits studied under the conditions and with the strain used in these experiments.It is pointed out that in the estimation of temperature coefficients comparison of the times taken to reach a constant result give more accurate and consistent values than comparison of the results reached in constant t