Environmental factors e.g. temperature, photoperiod, humidity, can play important roles in insect development. Many species use environmental cues to time events in their life and for orientation within their environment. Photoperiod and the physiological mechanisms of circadian rhythms are involved in regulating or inducing activities such as locomotion, oviposition, mating, metabolic rates, and diapause. Numerous studies have researched insects and their responses to photoperiod, including some that have looked at dipterans, but the literature does not clearly elucidate whether or not light alters development rates (Nabity 2005). Research with Calliphora vicina (Robineau-Desvoidy) has been conducted to examine how light is involved in diapause induction. The following research project examined the effects of photoperiod on development rates of Phormia regina (Meigen), Cochliomyia macellaria (Fabricius), and Calliphora vicina using four light regimes of 0, 12, 16 and 24 hours of light measured at both 20° and 26°C. Environmental growth chambers (Percival Scientific Inc., Perry, IA) at the University of Nebraska-Lincoln were used to conduct the experiments with larvae reared on ground beef or beef liver inside rearing containers. Using an ANOVA, the results showed significant differences in development rates, from egg to adult emergence (days) among several treatments for each species at each temperature. Photoperiod, while holding temperature constant, has an effect on larval growth rates of these 3 species of blow flies, and therefore will allow more accurate Post-Mortem Interval (PMI) determinations in medico-legal criminal investigations.