Bacillus thuringiensis subsp, israelensis (Bti), during sporulation, produces a protein crystal that is highly toxic to mosquito larvae. The Bti crystal contains three major proteins of 27 KDa 65 kDa and a doublet of 130 kDa. Four types of Bti toxin genes have been cloned in E.coli host. First

pMU388 was clones and characterized by Angsuthanasombat et al. (Mol.Gen.Genet.,1987). It contains 3.8 kb XbaI fragment of the Bti toxin gene and expresses the 130 kDa protein that is highly toxic to larvae of Aedes aegypti, but lowly toxic to larvae of Culex quinquefasciatus. Second

pMU500 was obtained from the same DNA library of pMU388. Its derivetive, called pMU500-2, consists of the 3.8 kb EcoRI-XbaI fragment of the Bti toxingene. E.coli/pMU500-2 produces the 125 kDa protein that is equally toxic to larvae of A. aegypti and C. quinquefasciatus. The two other clones, pMUEX3.6 and pMUPX3.8, contain a 3.6 kb XbaI-EcoRI and 3.8 kb PstI-XbaI fragments of the Bti toxin gene, respectively. These two toxin genes were cloned and screened by using 3 terminal of the PMU388 insert (corresponding to the carboxy half of the 130 kDa protein). E.coli/pMUEX3.6 produced a 58 Kda protein that showed significantly toxicity against larvae of A,aegypti, but insignificantly killed larvar of c, quinquefasciatus. E.coli/PMUPX 3.8 expressed a 65 kDa protein but did not exhibit mosquito larvicidal activity. A combination of two toxins, except that containing 65 kDa protein, showed synergistic toxicity. Probing plasmide of Bt. subsp. morrisoni strain PG14 with three of Bti toxin genes indicated that the plasmids of strain PG14 contained toxin genes related to those of Bti.