A spore-forming bacillus demonstrating a high level of larvicidal activity in nominally 12 hr. following challenge has been isolated from the screening of roughly 1,000 clones from some 10 soil samples taken at known mosquito larval breeding sites. Roughly 1 in 100 such clones demonstrated larvicidal activity of the same order of magnitude as Bacillus sphaericus var. Fusiformis (SSII-1) but only 1 isolate (60A) demonstrated rapid activity against species in 4 genera as evidenced by the 5 test species used, i.e., Anopheles sergentii (Theobald), Uranotaenia unguiculata Edwards, Culex univitattus Theobald, Aedes aegypti (Linnaeus), and Cx. pipiens Linnaeus. The activity of this isolate (60A) is some 30-100 times greater than observed using B. sphaericus (SSII-I), when assayed using Cx. pipiens test larvae. Successful screening was dependent upon the use of a proprietary nutrient N2X which was made available by Nutrilite. This media is a standard production media for Bacillus thuringiensis (HD-1). With only one exception (60A), all other toxic isolates did not demonstrate larvicidal activity when grown on nutrient agar (Difco). The isolate, 60A, demonstrated no significant loss in larvicidal activity after being heat-shocked for 20 min. at 60 C, lyophilized or exposed to ultra violet (2537 A), sufficient to reduce the viable spore count to less than 0.1% of its initial count. The mode of action can be attributed to an ultra violet and heat-stable endotoxin.