The period of insect activity (PIA) contributes information to estimates of the minimum postmortem interval in forensic investigations and begins with blow fly attraction and oviposition on a resource such as carrion or a human corpse. Incorrectly estimating nocturnal oviposition could alter PIA estimates by up to at least 12 h; however, the extent of this difference in PIA would depend on environmental and geographic factors. To date, the literature on the extent and frequency of blow fly nocturnal oviposition is conflicting. Our objectives were as follows: 1) to measure the effects of artificial lighting and beef liver bait height above ground on nocturnal and diurnal oviposition, and 2) to monitor oviposition through the night on swine carcasses exposed to the environment at dusk in different habitats over 3 yr. We documented no consistent nocturnal oviposition in any trial using beef liver or on carcasses in different habitats and seasons. There were statistically significant effects of light and height of bait above the ground on diurnal oviposition of Phormia regina (Meigen) in August of 2009, the only month with mean night temperatures >20°C. In August there also was significantly greater diurnal oviposition on liver bait placed on the ground compared with bait elevated 1 m. Our results suggest that nocturnal oviposition is rare in the natural environment. However, we found enhanced diurnal oviposition of P. regina under conditions of ambient temperatures >20°C the night before oviposition. Additional studies are needed to better understand the ecological mechanisms governing blow fly oviposition important to forensic entomology.