An outdoor test was conducted to evaluate initial efficacy and longevity of water-soluble pouches of Bacillus sphaericus (VectoLex WSP and VBC60035 WSP), applied as prehatch treatment against Culex species in simulated catch basins. Both VectoLex WSP and VBC60035 WSP, applied at 1 pouch (10 g) per basin (single treatment), yielded significant immediate and long-term (>90% for 203 days) control of late instars of Culex mosquitoes. Consistent and complete control of pupae (100%) as a result of larval mortality was clearly indicated for 70 days posttreatment. Control levels varied but remained high and significant on most sampling days afterwards. Exuviae counts also indicated complete control (100%) for 70 days posttreatment. Control levels indicated by exuviae counts, however, were not significant for most sampling days beyond this sampling day, because of low counts in the untreated controls. No significant differences were indicated in efficacy between VectoLex WSP and VBC 60035 WSP. The test was conducted under highly challenging conditions, such as prehatch treatment, highly polluted water, and frequent flushing. Spore counting in water and sludge samples verified the presence of B. sphaericus spores on day 196 posttreatment, after 28 flushes. The results strongly indicate that WSP formulation of B. sphaericus could be one of the best candidates for controlling larvae of Culex mosquitoes developing in catch basins, with significant initial and residual efficacy.