The possible impact of insecticides on post-mortem interval (PMI) estimations based on entomological evidence is a largely unexplored area of research. The purpose of this study was to determine insecticide toxicity to blowflies during egg, larval, and adult stages of development. Permethrin, a pyrethrin compound, was selected as the insecticide of study because of its high availability. Glass panes were treated with known concentrations of permethrin, or distilled water as a control, by spraying until run-off occurred. Blowfly larvae of each instar were then collected from field colonies and placed on the treated glass for one minute, after which they were transferred to chambers separated by treatment level and instar, supplied with a food source, and held in rearing cabinets (30C, 16:8 photoperiod, 75% RH). Mortality in larvae was measured 24 and 48 hours following exposure. Blowfly eggs were also tested, in a similar fashion, with percent hatch serving as the measured variable. Finally, adult blowflies were tested by placement in permethrin-treated vials, or water-treated vials as a control, with mortality measured after 24 hours. Permethrin toxicity, as measured by mortality, in eggs, larvae, and adults were statistically analyzed using Abbott’s corrected mortality formula and probit analysis. In future studies, the resulting toxicity levels will be used to determine the effects of permethrin on Calliphorid development over time, as well as to study how permethrin may affect PMI determination.