Experiments to determine the relationship between insecticidal droplet size and kill of adult mosquitoes were conducted using laboratory wind tunnel tests with monodisperse aerosols and field tests with ground ultra-low volume (ULV) aerosol generators. Wind tunnel tests with uniform sizes of malathion droplets produced from a Berglund-Lui Monodisperse Aerosol Generator were conducted with caged Aedes taeniorhynchus females. Mortality was determined for 3 exposure times for 18 droplet sizes in the range from 2.8 to 32.8 um. The results indicated that the optimum droplet size was in the range from 10 to 15 um in diameter and that insecticidal efficiency decreased rapidly for sizes smaller than 5 um and larger than 25 um in diameter, but little difference in efficiency was noted for sizes from ca. 7 to 22 um. Field tests were conducted with malathion aerosols having a range of droplet sizes produced by ground ULV generators operated to provide volume median diameters (VMD's) of 5, 10, 15, 24 and 39 um. Caged Ae. taeniorhynchus and Anopheles quadrimaculatus females were exposed in an open field under very selective atmospheric conditions. The results indicated that the 10 and 15 um VMD aerosols were most efficient with 82% mortality each, compared to 67 and 72% mortality for the 5 and 24 um VMD's. The 39 um VMD aerosol was clearly less efficient that the smaller aerosols with a comparative mortality of 33%. The results of the field tests were consistent with laboratory tests.