A test challenge procedure has been outlined, using 3rd instar Culex pipiens larvae held in a defined aquatic microbial test flora AS 2(7), which provides for a standardized bioassay of the entomotoxic activity per cell of Bacillus thuringiensis var. israelensis (Bti). AS 2(7), a selected soil microbial isolate, following inoculation and incubation with a sterilized Tetramin suspension in dilute alfalfa infusion, provides an excellent mosquito larval nutritional food source as evidenced by an 89% adult emergence rate. Since the rate of mosquito larval filter feeding is nominally 10 ml/hr, a larval test challenge using 2 larvae held in 4 ml of fluid results in near complete larval filter feeding of a test challenge within less than one hour. As a consequence, a larval test challenge can be directly expressed in terms of ingested dose. Using the outlined standardized larval test challenge procedure a comparative evaluation of several commercial formulations of Bacillus thuringiensis var. israelensis (Bti), both liquids and wettable powders, has been reported.