The larvicidal activity of spinosad was assessed against the house fly, Musca domestica L. (Diptera: Muscidae) and the stable fly, Stomoxys calcitrans (L.) under laboratory conditions. Fly rearing media was spiked with known concentrations of spinosad, spanning 0.5 – 4.0 ppm for house fly experiments and 1.0 – 5.0 ppm for stable fly experiments. Stable fly larval rearing media consisted of fermented Purina fly media, while house fly media consisted of wheat bran and calf manna. Newly oviposited fly eggs were placed onto spiked media and incubated at 23 degrees C and 63% relative humidity until post-pupation, approximately 10 days for house flies and 14 days for stable flies. After pupation, the numbers of pupae and/or adult flies were assessed. The EC50 for house flies was 0.6331 ppm (R2=0.9239, 95% CI=0.1713 to 2.340). Data from stable fly experiments will be also be presented. Our results suggest that spinosad may be of potential utility as a larvicide to aid in the control of nuisance and biting flies.