Laboratory experiments were conducted to determine the effects of Bacillus thuringiensis var kurstaki on larvae of the tobacco budworm (TBW), Heliothis virescens (F.). Larvae were exposed to Bt in artificial diet for 3-5 days and the effects of Bt treatment on growth and development were measured. We also measured the effect of acidity and alkalinity on activity of Bt. In other experiments we compared the performance of an old sample of Dipel® to a new sample of Dipel® and the activity of three different formulations (EC, WP and engineered Bt.) was compared. Exposure to B.t. greatly slowed the development of larvae at concentrations as low as 2 parts per trillion, but did not produce much mortality at that level. Alkaline pH provided more growth reduction and mortality compared to acidic pH. Old Dipel® sample provided more growth inhibition and mortality compared to a newer sample. Engineered Bt produced more mortality whereas the other two formulations exhibited more growth reduction ability. The data suggest that exposure to low rates of Bt is slowing down the development of TBW larvae. Hence Bt at low dose rates may be useful in managing TBW. By using Bt at low rates throughout the season the development of larvae will be slowed down, making them available for the beneficials to feed on. Data also suggest that the major effect of Bt may be as an insect growth regulator rather than as an insecticide. By using low doses we may be able to reduce the selection pressure for resistance and by incorporating with low doses of insecticides if necessary, TBW can be managed successfully.