A large number of source materials were collected for isolating entomopathogenic bacteria from larval mosquito habitats in Kirinyaga District, Kenya. Mosquito-toxic bacteria were included among the numerous types of microorganisms isolated from the habitats. The pathogenic isolates shared common structural characteristics; they were gram-positive, spore-forming bacilli that produced parasporal inclusions conferring broad-spectrum larvicidal activity against Anopheles, Culex and Aedes mosquitoes. Based on structural and growth characteristics, coupled with larvicidal activity, the pathogenic isolates were tentatively identified as variants of Bacillus thuringiensis. Although the collection consisted of a variety of items including soil, silt and mud, the most productive materials were larval bodies. Using healthy mosquito larvae held in a fully permeable plastic bottle, a baiting technique was developed as a means of recovering bacteria from the environment.