Bacillus thuringiensis var. israelensis (WHO/CCBC 1897) was shown to produce 100% mortality at a dosage of 1 x 105 bacilli/ml in laboratory studies against six species (in 3 genera) of mosquito larvae commonly found in California. Tests were conducted in water from natural breeding sources which included treehole and brackish sites. Neither different water qualities nor full sun exposure appeared to diminish the activity if the pathogen under the conditions of these tests.