The city of Chicago used ground ultra-low volume treatments of sumithrin (ANVIL 10 10) in areas with high West Nile virus infection rates among Culex mosquitoes. Two sequential treatments in Morbidity and Mortality Weekly Reports wk 31 and 32 decreased mean mosquito density by 54% from 2.5 to 1.1 mosquitoes per trap-day, whereas mosquito density increased by 153% from 1.3 to 3.3 mosquitoes per trap-day at the nonsprayed sites. The difference between these changes in mosquito density was statistically significant (confidence intervals for the difference in change: −4.7 to −1.9). Sequential adulticide treatments in September (wk 34 and 35) had no effect on mosquito density, probably because it was late in the season and the mosquitoes were presumably entering diapause and less active. Overall, there was significant decrease in mosquito density at the trap sites treated in all 4 wk (wk 31, 32, 34, and 35), suggesting that sustained sequential treatments suppressed mosquito density. Maximum likelihood estimates (MLE) of infection rate estimates varied independently of adulticide treatments, suggesting that the adulticide treatments had no direct effect on MLE. Mosquito trap counts were low, which was probably due to large numbers of alternative oviposition sites, especially catch basins competing with the gravid traps.