The effectiveness of the upstream point treatment by AQUABAC® Biolarvicide (Bacillus thuringiensis var. israelensis, Biological Larvicide Aqueous Suspension) against Culex quinquefasciatus Say in slow-flowing water ditches has been evaluated in St. Augustine Beach, St. Johns County, FL. The upstream point treatment by AQUABAC (liquid) at 0.5 liter/acre provided significant levels of larval control of Cx. quinquefasciatus for all sections of the downstream for 9 days. The upstream point treatment method by larvicides may have potential for use, minimize environmental impacts, and reduce cost and labor in mosquito-control operation for slow-flowing water ditches.