The heat resistance of Bacillus subtilis var. niger has been measured from 85 to 125 degrees C using moisture levels of percent relative humidity (%RH) less than or equal to 0.001 to 100 in a closed system. Five curves have been presented to characterize the thermal destruction, using thermal death times defined as F values at a given combination of three moisture and temperature conditions. Reductions of 99.99% (4-log10 cycles) of the initial population were estimated for the three moisture conditions. At 110 degrees C, the expected time for a 4-log10 reduction was 1.1 h at %RH = 100, 3.1 h at %RH less than or equal to 0.1 and 54 h at %RH = 10.7. Goodness-of-fit tests to examine the adequacy of three polynomial models failed to indicate a trend. The linear model (from which estimates of D are obtained) was satisfactory for estimating the thermal death times (%RH less than or equal to 0.1) in the plate count range. The estimates based on observed thermal death times and D values for the %RH = 100 diverged so that D values generally gave a more conservative estimate over the temperature range 90 to 125 degrees C. Estimates of ZF and ZL ranged from 32.1 to 58.3 degrees C for the %RH less than or equal to 0.1 and 100. A ZD value of 30.0 was obtained for data observed at %RH less than or equal to 0.1. The ZF results were obtained from plotting observed log times to achieve a 99.99% reduction in the initial population versus temperature. Estimates of ZL and ZD were obtained by using linear estimates of L100 approximately equal to 4D and D values in a similar plot.