With the advent of West Nile virus in the US in 1999, mosquitoes have become a significant vector of disease across the US. Local mosquito control efforts have increased since that time, but many control methods have undesirable consequences for non-target organisms. Biological control is potentially a mosquito control method with the least environmental impact, yet the effectiveness of this method has rarely been assessed in large-scale field applications. In this study, the effectiveness of aerial larvicide applications with Bacillus sphaericus and B. thuringiensis israelensis was assessed comparing adult mosquito trap counts post application with historical data obtained in San Diego County from the Vector Control program. Aerial sites were chosen based on size, mosquito breeding potential, inaccessibility, and proximity to population centers. A helicopter was used to apply monthly treatments of B. sphaericus (VectoLex®) and B. thuringiensis israelensis (VectoBac®), at a ratio of 2:1. Mosquito counts were obtained through bi-weekly surveillance using CDC traps. Culex erythrothorax was used as an indicator due to its historical abundance during May through October. Results show a decrease in the C. erythrothorax population that was significant at four of the five monitored sites (overall median post biological control adult C. erythrothorax was 2, range 0 - 72, as compared to median 169, range 5 - 540, during the same time of year from 1998-2002, p<0.05). Mosquito trap data confirm aerial larvicide applications effectively reduced C. erythrothorax populations and maintained lower mosquito numbers throughout the mosquito breeding season, indicating that control of mosquitoes can be accomplished through the use of biologically-based larvicidal applications.