For biocontrol of mosquitoes, mosquitocidal toxin genes from Bacillus thuringiensis subsp. israelensis and Bacillus sphaericus have been cloned into a number of cyanobacteria. However, little is known about the persistence of such recombinant cyanobacteria in mosquito larval habitats. Four fresh water unicellular cyanobacteria, Synechococcus PCC6301, PCC7425, PCC7942 and Synechocystis PCC 6803, were evaluated under laboratory conditions related to mosquito breeding environments, Results indicated that Synechococcus PCC6301 was potentially the most suitable organism for use in the natural mosquito habitat as it could tolerate a wide range of temperatures, salinities, and biological and chemical insecticides. Moreover, strain PCC6301 could be ingested and digested by Culex quinquefasciatus larvae and could support the development of larvae to full insect maturity.