Techniques were developed to use a Coulter Counter (Model TA) to measure automatically the droplet size distribution of insecticidal aerosol samples collected in a liquid medium in the field. Samples of aerosols generated by ultralow volume (ULV) ground equipment were collected in a settlement chamber. Petri dishes (100 mm diameter) containing the collection liquid were placed on the chamber floor. A collection liquid consisting of a 45% sucrose/Isoton solution plus 0.6% Nacconal 90F as a surfactant was found to be satisfactory for technical malathion (diethyl) mercaptosuccinate S-ester with O,O-dimethyl phosphorodithioate) (95%) and other insecticides with similar physical properties (specifically, technical fenthion (0,0-dimethyl O-[4-methylthio)-m-tolyl] phosphorothioate) and fenitrothion (O.O-dimethyl O-(4-nitro-m-toly) phosphorothioate). A settlement time of 15 min was found to be adequate to obtain an aerosol sample. Droplet size measurements with the coulter counter were in close agreement with standard microscopic measurements of droplets collected by settlement onto Teflon coated glass slides. For 11 treatments with a malathion aerosol atomized at 4 psi, the volume median diameter (VMD) for microscope measurements averaged 18.7 pm (3.1 pm standard deviation) and accompanying coulter readings averaged 18.2 pm (0.8 pm standard deviation.) The procedures developed in this work provide for automatic measurement of droplet size distributions that can be used to facilitate extensive research on aerosol generators and the effects of different size distributions on insecticidal effectiveness.