Using synthetic oligonucleotides designed on the basis of the binary toxin gene sequence of 2362, a 1.1 kilodalton DNA fragment has been prepared as a probe to study the toxin genes of several Bacillus sphaericus strains. The 3.5kb Hind 1 DNA fragments of C3-41, 1AB881, IAB872 and BS-197 as well as the 4.7kb Hind 11 DNA fragment of LP1-G are homologous to the probe. The 51.4 and 41.9 kilodalton binary toxin can be detected by SDS-PAGE and Western Blotting. The results of the bioassay showed that C3-41, IAB881, BS-197 and 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 LP1-G have medium toxicity against the resistant larvae.