Laboratory bioassays were conducted on the efficacy of a water-dispersible granule (WG) formulation of Bacillus thuringiensis variety israelensis (VectoBac WG; active ingredient [AI]: 3,000 Bti international toxic units [ITU]/mg) against third instars of six common Australian mosquito species, Aedes aegypti (L.), Ochlerotatus vigilax (Skuse), Ochlerotatus notoscriptus (Skuse), Culex sitiens Wiedemann, Culex annulirostris Skuse, and Culex quinquefasciatus Say. The normal model for log-linear mortality data was used to determine laboratory 48-h LC₅₀ and LC₉₅ values. The target mosquito species tested were extremely sensitive to the VectoBac WG formulation, with the most sensitive species (Cx. annulirostris and Cx. quinquefasciatus, LC₉₅ value of 0.019 ppm) being twice as susceptible as the most tolerant (Oc. notoscriptus, LC₉₅ value of 0.037 ppm). Cx. annulirostris was selected as a target species for a small-plot evaluation of VectoBac WG and VectoBac 12 aqueous solution (AS) ([AI]: 1,200 Bti ITU/mg) efficacy over time, in freshwater in southeastern Queensland, Australia. Replicated cohorts of caged third instars were exposed weekly to six concentrations of WG formulation (0.004–0.13 ppm) and three concentrations of the 12AS formulation (0.04–0.13 ppm). In water with high organic content, treatment concentrations of 0.008 ppm WG and 0.04 ppm 12AS and above produced significant larval control (≥96%) at 48 h posttreatment, with no residual control at week 1. Water quality was not affected by treatment with either formulation.