A series of laboratory studies was conducted to assess the effect of droplet size on efficacy of pyrethrin aerosol against adults of Tribolium confusum Jacqueline DuVal, the confused flour beetle. A vertical flow aerosol exposure chamber that generated a standardized particle size diameter was used for these trials. In the first experiments, adults were exposed in the chamber for 2.5–45 min to aerosol dispensed at a volumetric median particle size diameter (VMD) of 16 µm, and then held in the arenas in which they were exposed or transferred to new arenas with or without a flour food source. All adults were initially knocked down when removed from the chamber. Recovery from knockdown decreased as exposure interval increased, but the presence of a food source enhanced recovery at the lower exposure intervals. In the second experiment, the aerosol was applied at a VMD of 2 µm and adults were exposed for between 5 and 75 min. Knockdown of adults was ≤10% when adults were removed from the chamber regardless of exposure time and afterward there was essentially complete recovery of adults. In the third and final experiment, the same 2-µm VMD particle size and exposure times were used, but the concentration of aerosol was increased by ≈4× compared with the previous experiment. In this test, initial knockdown was greater at the higher exposure intervals, but by 3 and 4 d posttreatment, recovery was again essentially 100%. This is the first published test assessing the efficacy of specific aerosol particle sizes on a stored product insect. Results indicate that particle size was a more important factor in conferring toxicity than the actual concentration or number of aerosol particles.