Forensic entomology is an important field of forensic science that utilizes insect evidence in criminal investigations. Blow flies (Diptera: Calliphoridae) are among the first colonizers of remains and are frequently used in determining the minimum postmortem interval (mPMI). Blow fly development is influenced by a variety of factors including temperature and feeding substrate. Unfortunately, dietary fat content remains an understudied factor in the development process, which is problematic given the high rates of obesity in the United States. To study the effects of fat content on blow flies we investigated survivorship, adult weight, and development time (overall and by sex) of Lucilia sericata (Meigen) and Phormia regina (Meigen) on ground beef with 10%, 20%, or 27% fat. As fat content increased, survivorship decreased across both species with significant impact to P. regina. While P. regina adults were generally larger than L. sericata across all fat levels, only L. sericata demonstrated a significant (P < 0.05) difference in weight by sex. Overall development times did not vary by fat content, excluding 27% for P. regina. Additionally, development times did not vary by sex for P. regina but did differ (P < 0.05) for L. sericata with females taking longer to develop. These findings provide insight into the effect of fat content on blow fly development, a factor that should be considered when estimating an mPMI. By understanding how fat levels affect the survivorship and development of the species studied here, we can begin improving the practice of insect evidence analysis in casework.