A laboratory protocol is presented for the bioassay and efficacy testing of primary powders and formulations of Bacillus thuringiensis var. israelensis (serotype 14). The recommended bioassay apparatus is a modification of that designed by S. Hembree and co-workers. It utilizes an electric motor to drive a series of belt-connected pulleys, and a shaft, connected to each pulley, is attached to a plastic bottle which provides current within each bioassay container. After 3 hr of acclimation in the bioassay containers, 25 penultimate instars of the species to be tested are exposed for 15 min to the appropriate concentration of B. thuringiensis, after which time the material is flushed from the bioassay container through a drain tube. Mortality is scored at 48 hr posttreatment. Nonchlorinated water containing 5 ppm of finely-ground larval diet (rabbit chow) is recommended as the test and posttreatment medium. Although 20°C is proposed for testing, cooler temperatures may be required for some species from temperate climates. The IPS-78 B. thuringiensis var. israelensis standard is recommended for susceptibility testing as well as comparison with other isolates, primary powders and formulations. Treatment concentrations of B. thuringiensis should be expressed as wt/vol and spores/ml. For calculation of LC50-95 a minimum of 5 data points within the 10-95% mortality range is suggested.