Abstract: The mosquito specific cry 11A gene of Bacillus thuringiensis subsp.israelensis was cut out from pGEM1 with BamHI and ligated into the BamHI site of a E. coli-cyanobacteriun shuttle vector pRL25C. The recombinant plasmid was transformed into E. coli HB101 which bears plasmid pRL528, a helper plasmid carrying mob and genes for Eco47II and AvaI methylase. Then three parents: E. coli HB101 (bearing the recombinant plasmid and helper plasmid pRL528), E. coli HB101 (bearing RP4) and Anabaena PCC7120 (a filamentous cyanobacterium which can fix nitrogen and can be fed by mosquito larvas in water) were put together to do triparent matings. Both Southern blot and Western blot demonstrated that the recombinant plasmid existed and expressed in the positive conjugant of Anabaena PCC7120. But the expression of the cry 11A gene with its original promoter in transgenic Anabaena PCC7120 was too low to kill mosqinto larvae.