This paper describes a method for separating plant mortality due to heat from mortality caused by herbage removal; squirreltail (Sitanion hystrix (Nutt.) J. G. Smith) and needle-and-thread (Stipa comata Trin. and Ruper.) are used as examples. After plants are burned in the field under controlled conditions, ratios are calculated between field-measured durations of temperatures in plants and laboratory-measured thermal death times for plant tissue. If the sum of the ratios for the various temperatures is 1.0 (designated 1 necrotherm) or greater, mortality is attributed to heat; if the sum is less than 1.0, to herbage removal. Further testing of the "predictor" on more species, sites, years, etc., is needed, but the method appears very promising. For the two bunchgrasses tested, heat alone accounted for all mortality by fire, except during July. The time required for a specific temperature to kill plants varied with season. To predict thermal death times for bunchgrasses and thus simplify the laboratory determinations, an empirical equation was developed.