Insecticides are the primary means to control Culex pipiens, an enzootic vector of West Nile virus, in the USA. To better understand how the evolution of resistance might impact control of this insect, we investigated the levels of resistance in populations collected from 2 metropolitan areas (Albany and Syracuse, NY) to 4 larvicides (methoprene, phenothrin, Bacillus sphaericus [Bs], and Bacillus thuringiensis israelensis [Bti]) and 1 adulticide (phenothrin) registered for mosquito control in New York State. High levels of resistance were found only to Bti, and only at 1 site (Syracuse). Resistance levels to the other insecticides were less than 10-fold. Given the large difference in Bti resistance between Syracuse and Albany, it appears these populations of Cx. pipiens do not rapidly mix, leading to localization of resistant populations.