Genetic engineering techniques have been used to significantly improve mosquito larvicides based on the bacteria Bacillus thuringiensis (Bt) subsp. israelensis (Bti) and Bacillus sphaericus (Bs). These new larvicides hold excellent promise for providing better and more cost-effective control of nuisance mosquitoes and vectors of important diseases, including the anopheline vectors of malaria and culicine vectors responsible for filariasis and viral encephalitides. The toxicity of Bti and Bs is due primarily to endotoxin proteins produced during sporulation. After ingestion by larvae, these are activated and destroy the larval stomach, quickly resulting in death. By cloning the genes encoding various endotoxins from Bt and Bs species, and engineering these for high levels of synthesis, we have been able to generate recombinant bacterial strains based on Bti that are more than 10 times as effective as the conventional strains of Bti or Bs that serve as the active ingredients of commercial bacterial larvicides currently used for mosquito control. The best of these recombinants contain all major Bti endotoxins, specifically, Cry4A, Cry4B, Cry11A, and Cyt1A, plus the binary (Bin) endotoxin of Bs, the principal mosquitocidal protein responsible for the activity of this species. The presence of Cyt1A in these recombinants, which synergizes Cry toxicity and delays resistance to these proteins and Bs Bin, should enable long term use of these recombinants with little if any development of resistance. In the field, these new recombinants should be particularly effective larvicides against most important vectors and nuisance species of the genus Culex, the malaria vectors Anopheles gambiae and An. arabiensis, and species of Aedes and Ochlerotatus sensitive to Bs.