A study to determine the effects of machine pressure and insecticide flow rate on the size of aerosol droplets as they relate to the Cythion label was conducted with 3 different ground ULV aerosol generators. An increase in flow rate required a corresponding increase in blower pressure to maintain the labeled droplet mass median diameter of 17 microns or less. Droplets larger than 48 microns were frequently sampled at machine pressures less than 6 psi (41.4 kPa). At the highest labeled flow rate of 8.6 fl oz/min (254.3 ml/min), machine pressures of 7-8 psi (48.3-55.2 kPa) were required for each of the 3 aerosol generators tested to consistently conform to the droplet criteria of the Cythion label.