The effectiveness of Bacillus thuringiensis var. galleriae HD-129 against the moths and egg masses of the cotton leafworm, Spodoptera littoralis, has been demonstrated in laboratory experiments. The longevity and egg production were significantly affected when the moths were fed on a sucrose diet containing 17.2 × 103 IU of the tested formulation/mg or higher rates. At lower concentrations (4.4−8.7 × 103 IU/mg), the females gave normal egg production, but the longevity was adversely affected. Investigations show that a formulation with spores alone affects the egg production and hatching, while a formulation with active crystals alone had no effect on egg production. Sprays of B. thuringiensis combined with sucrose on flowering cotton plants seem to affect the biology of the moths released on it. Egg masses sprayed with B. thuringiensis hatched normally, but the survival of the hatched larvae was reduced when treatment was made shortly before egg hatching. Based on the results obtained, B. thuringiensis showed a promising effect in the control of the moth S. littoralis as well as an ovicide larvicide agent.