An ecological study to evaluate necrophagous species richness, abundance, and diversity was conducted in four contrasting ecoregions of Los Angeles County, California, United States: a highly anthropized area, two moderately populated areas, and a sparsely populated area. Our hypothesis states that there will be higher abundance of exotic species in greatly anthropized ecoregions and higher diversity of native calliphorid flies in sparsely populated sites. A total of 2,883 blow flies belonging to 12 species were captured from the four ecoregions. The exotic Chrysomya rufifacies (Macquart) (Diptera: Calliphoridae) was the most abundant species, representing 72.84% of all Calliphoridae collected. The highest abundance was in the highly anthropized ecoregion. Differences of the blow fly assemblages across environmentally and anthropically different ecoregions were found, and that the relative abundance of the exotic species increased with anthropization. Our results suggest that both anthropization and elevation alter the structure of blow fly assemblages, modifying the abundance and species richness of Calliphoridae. The results of this study support the hypothesis of biotic homogenization, since we found a higher proportional abundance of exotic species and lower diversity of native blow flies in ecoregions heavily modified by human action. Additionally, the hypothesis of intermediate disturbance was supported as the highest diversity was found in the ecoregion with moderate anthropization mixed with natural elements. This is the first study to conduct an intensive survey of the Calliphoridae assemblages in Los Angeles County, which can be potentially used as a tool in human and veterinary health and forensic sciences.