A field trial using the entomopathogen Bacillus sphaericus against Culex quinquefasciatus was conducted in a district of Recife, Brazil, an area with a high prevalence of lymphatic filariasis. In this urban area more than 2,500 Cx. quinquefasciatus breeding sites within a 1.2-km2 area were found and subsequently submitted to a long-term treatment with B. sphaericus. To evaluate the impact of treatment on the densities of Cx. quinquefasciatus preimaginal forms, 26 breeding sites, representative of the major site types found in that area, were monitored for 3 years. Parameters such as mean and maximum densities of larval population as well as the frequency of infested sites were recorded before, during, and after the trial. The level of each parameter was greatly reduced during the treatment period, particularly in the 2nd treatment year. A low density of Cx. quinquefasciatus was also maintained throughout the last year, in the absence of treatment. Analysis of the data reported here confirms B. sphaericus as a very effective larvicide against Cx. quinquefasciatus despite the optimal environmental conditions for mosquito proliferation in the urban area of Recife.