The Beecomist spray head was evaluated for aerial application of Bacillus thuringiensis var. israelensis (serotype H-14; Bti) at various ultra low volume (ULV) rates against natural populations of Anopheles quadrimaculatus larvae in rice fields. Deposits on Kromekote cards indicated that 0.54 liter/ha of neat Bti penetrated the dense canopy of the rice field. Mean number of droplets 65 cm below canopy level was 4.9 +/- 5.0/100 cm2. At 1 day posttreatment, applications of 0.54, 0.27, 0.11, 0.07 and 0.04 liter of Bti/ha resulted in reductions of 97.9, 94.4, 93.0, 71.1 and 21.8%, respectively, of An. quadrimaculatus larvae/dip. Calculated lethal field dosages (LFD50 and LFD90) were 0.05 and 0.16 liter/ha, respectively.