Spray drift determinations of truck-mounted ULV applications of temephos and Bacillus thuringiensis var. israelensis (B.t.i.) in simulated field trials were conducted in the Baix Llobregat area near Barcelona, Spain, using larval Culex pipiens as bioindicators. A 50:50 temephos:oil mixture applied at 350 ml/min, while traveling 9 kph, resulted in 100% control of larvae 10-50 m from the point of application (with the exception of 82% mortality at 30 m); control decreased rapidly to 50% at 55 m. Mortality due to B.t.i. application ranged from 92 to 100% within 12 m, and generally decreased therafter to about 11% at 30 m.