Technical Abstract: It has been commonly assumed that arthropod species living or hibernating in the soil would not be affected by grassland fires, even though grassland fires result in elevated surface and belowground soil temperatures. The importance of elevated belowground soil temperatures occurring during grassland fires on the survival of grasshopper eggs had not been examined. The effects of simulated fall grassland fires of varying intensities on egg mortality were examined using a grasshopper species which lays shallow egg pods (Ageneotettix deorum) and a species laying deeper egg pods (Melanoplus sanguinipes) to test the hypothesis that exposure to heat during rangeland fires was the mechanism responsible for population reductions in A. deorum following fire. Species specific oviposition characteristics mediated the effects of fire intensity on belowground egg mortality. The results indicate that fires occurring in areas with at least 3100 kg ha-1 standing crop biomass would significantly reduce populations of A. deorum, but not M. sanguinipes. No A. deorum eggs hatched in 12 of the 14 oviposition containers subjected to simulated fires approximating a standing crop biomass of 4500 kg ha-1. This is the first study to link field observations of rangeland insect populations following fire to mechanisms related to belowground egg mortality.