In a laboratory investigation, the effect of spray droplet size on the mortality of three life stages (eggs, nymphs, and adults) of the Asian citrus psyllid (ACP), Diaphorina citri Kuwayama, was determined. Six spray droplets of nearly uniform size (diameter) (D¯₁₀ = 40.5, 52.0, 101.5, 147.9, 174.0, and 264.7 μm) of fenpropathrin, a synthetic pyrethroid, were generated using a piezo-electric droplet generator. The formulation active ingredient (AI) was applied at spray delivery rates of 0.38 or 0.58 mg AI/min (168.9 and 253.4 g AI/ha), which were equivalent to 1/2 and 3/4 of the recommended field rates, respectively. The desired rates were achieved by moving the target plants across the spray stream at speeds of 7.2 or 27.2 cm/s. These operating parameters resulted in a high droplet density of 81.3 to 327.2 droplets/cm² and a low droplet density of 20.8 to 130.7 droplets/cm². The ACP populations were counted at 3 and 7 days after spray (DAS). In general, the percent mortality of ACP eggs, nymphs, and adults decreased at 3 and 7 DAS as the droplet size increased, regardless of the spray discharge rate or droplet density. Smaller droplet sizes (40.5 and 52.0 μm) resulted in significantly greater ACP mortality than the remaining droplet sizes, with few exceptions. The intermediate droplet sizes (101.5 and 147.9 μm) resulted in ACP mortalities ranging from 40 % to 92 %, and larger droplets (174.0 and 264.7 μm) resulted in the lowest mortality at both droplet densities. These laboratory results suggest that field applications of fenpropathrin with smaller droplet sizes might be more effective against ACP than applications with larger droplets.