The relative efficiency of Bacillus thuringiensis formulations in aerial applications against Thyrinteina amobia (Lepidoptera: Geometridae) was evaluated in this essay in comparison with deltamethrin. The essay was carried out in Eucalyptus urophylla plantation in Monte Dourado, Fará, Brazil, through aerial application in areas of 10 ha (100 x 1000 m) for each treatment involving commercial formulations of B. thuringiensis and/or deltamethrin. Data were collected in five sample points for each treatment, 24, 40, 72. 96 and 120 hours after the insecticide applications and after 144 hours a drastic spraying with malathion and deltamethrin was made in order to know the total insect number in each sample point. Deltamethrin showed a faster effect over the caterpillar populations and higher efficiency at 200 and 400 ml/ha. The action of B. thuringiensis was slower, but high quality formulations of this bacteria seem to be promising against caterpillars in reforestation areas of the Amazon region, and can be used isolated or in mixture with deltamethrin for a faster effect. Deltamethrin besides B. thuringiensisshowed little effect on predators Hemiptera and because of this it can be applied against eucalyptus caterpillars, preserving their natural enemies.