Three indigenous isolates of Bacillus sphaericus (ISPC-5, ISPC-6 and ISPC-8), along with standard 2362 and 1593 strains, were evaluated for spore viability and mosquitocidal activity. Among these, ISPC-8 was the most viable and virulent isolate, exhibiting a significantly higher total viability count (TVC) and lower LC50 values. The TVC of the standard strains ranged from 4.0 to 9.2 x 10⁸ spores mL^-1, whereas it was 1.3 x 10⁹ spores mL^-1 for ISPC-8. The LC₅₀ values of ISPC-8, 2362 and 1593 against Culex quinquefasciatus were 0.68 x 10³, 1.22 x 10³ and 1.85 x 10³ spores mL^-1, respectively. The ISPC-8 was further assessed for host spectrum and found to be more active against C. quinquefasciatus, followed by Culex tritaeniorhynchus, Aedes albopictus and Aedes aegypti. The ISPC-8 strain was thus found to be a promising isolate for developing biopesticides. Among the indigenous strains, only ISPC-8 was found to have binary toxin genes (binA and binB). Comparative sequence analysis revealed that the BinA (41.9 kDa) protein of ISPC-8 differs by one amino acid (R197M), whereas BinB (51.4 kDa) differs by two amino acids (H99P, P174S) as compared with 1593 and 2362 strains. The purified binary proteins of ISPC-8 showed an LC₅₀ value of 6.32 ng mL^-1 against C. quinquefasciatus larvae after 48 h.