Using synthetic oligonucleotide primers designed on the basis of the binary, toxin gene sequence of 2362, a 1.1 kb DNA fragment has been produced by, PCR and prepared as a probe to detect the toxin genes of several Bacillus sphaericus strains. The 3.5kb Hindlll DNA fragment Of C3-41, IAB881, IAB872 and BS-197 as well as the 4.7 kb HindIll DNA fragment of LPI-G are homologous to the specific probe. All strains could produce the 51.4 and 41.9 kilodalton binary toxins assembled as crystals during their sporulations, which has been proved by SDS-PAGE and Western Blotting. The results of the bioassay showed that C3-41, IAB881, BS-197 land the reference strain 2362 have a high potency, against susceptible Culex pipiens subsp. pipiens and no potency against the resistant mosquito larvae from France(SPHRAE). IAB872 and LPI-G have medium toxicities against the resistant larvae.