Optimum flight path interval of Beecomist-applied Bacillus thuringiensis var. israelensis (serotype H-14) (Bti) against Anopheles quadrimaculatus larvae was determined by assessing the effective swath width in rice fields. Droplet sensitive cards, laboratory-reared and naturally occurring populations of larvae were used to monitor aerial treatments 1 day posttreatment. Overlapping swaths were necessary to provide high levels of larval reduction. Based on tests where flight path intervals were 18.3, 36.6 and 73.2 m, optimal flight path interval was estimated to be approximately 67 m downwind of the extreme downwind flight path when flow rate was 1.44 liter/min.