Field tests of ground ULV application of Cythion diluted with heavy aromatic naphtha (HAN), 1:4 ratio, at 0.45 oz AI/acre resulted in a 96% and 49% mortality of caged adult Aedes taeniorhynchus located 150 and 300 ft downwind. Cythion, 0.45 oz AI/acre produced a 33% mortality at 150 ft and a 21% mortality at 300 ft. This compares with an 8% mortality at 150 ft on a plot treated with HAN only (0.45 oz/acre), and a 6% mortality in an untreated area. The study was repeated on four successive nights, reversing test plots, and the data pooled. Statistical analysis shows that the toxicity of the active ingredient, malathion, is significantly increased with the addition of the diluent, HAN.