Nine bacterial strains isolated from different insect hosts were identified by using Sherlock Microbial Identification (MIS) system. Cell suspensions of the bacteria were tested for their larvicidal activity against the larvae of Culex pipiens L. (Diptera: Culicidae) under laboratory conditions. Fatty acid methyl esters (FAMEs) profiles and profiles of nutrient substrate utilization by the most effective strains were determined. Nitrogen fixation, antibiotic resistance and salt tolerance of these strains were also investigated. Separately, they were tested for toxicity in non-target aquatic organisms, including common water fleas, Daphnia magna and Nile tilapia (Oreochromis nilotious L.). Tested strains were identified as Bacillus thuringiensis (6 strains), Bacillus sphaericus (2 strains) and Brevibacillus brevis (1 strain). The identification was also confirmed by BIOLOG system. A mortality of 100% of Culex larvae was observed after 24 and/or 48 ins, following treatment with the cell suspension of Brevibacillus brevis FD1, B. sphaericus FD48 and B. sphaericus FD49 strains. Major fatty acids of B. sphaericus FD48 and FD49 strains were C15:0 iso, C15:0 anteiso and C16:1 w7c alcohol. Major fatty acids of the B. brevis FD1 strain was C15:0 anteiso, C15:0 iso and C17:0 anteiso. B. sphaericus FD48 and FD49 strains had similar substrate utilization patterns. Methyl pyruvate was the only substrate utilized by FD1, FD48 and FD49 strains. The strains grew in N-free Dobereiner culture medium, toleranced to salt and were resistant to amikacin, kanamycin, penicillin, tobramycin, ofloxacin and sulphamethoxazole. B. brevis FD1, B. sphaericus FD48 and FD49 whole culture gave an LC50 of approximately 10(3) cells/ mL for susceptible C. pipiens larvae, whereas a concentrated 10(9) cells/ml culture had not affected any in common water fleas and Nile tilapia survival. No mortality was observed throughout the 3-week experiment. In conclusion, B. brevis FD1, B. sphaericus FD48 and FD49 strains can be developed and recommended for biocontrol of mosquitoes.