Studies were conducted in mountainous areas of California and Oregon to test the effectiveness of Bacillus thuringiensis serotype H-14 in controlling larvae of snow pool Aedes mosquitoes, and also the effect of such larval control on the density of adult mosquitoes. California and Oregon studies showed that a wettable powder formulation of B.t.i. was effective in controlling mosquito larvae. In Oregon, treatment was effective even at water temperatures as low as 5 degrees C. Sampling of adult mosquitoes at campgrounds where intensive larval control was done failed to demonstrate a lowering of adult mosquito density in comparison with untreated control areas. Flight range studies showed that snow pool Aedes species can fly distances of up to 2 km, suggesting that infiltration from outside the treated area was one cause of failure. Inadequate seasonal timing appeared to be an additional factor. Using a simple computer model, we estimated that larval mosquito control with B.t.i. could be done for about $5,000 per season for a 500 ha area similar to our test area, a cost of $10 per ha ($4 per acre).