Twenty-eight strains of Bacillus thuringiensis (Bt) were screened for toxicity against the diamondback moth (DBM), Plutella xylostella. Bt strains were cultured on agar plates, examined for the presence of crystals and then harvested in water. Samples of Bt whole culture preparations, those treated with base [to solublize the crystal protein (pH-treated Bt)], as well as isolated spores were applied to artificial diet prior to placing 2nd instar DBM larvae on the diet. The soluble protein concentration was 2 or 3 times higher in Bt preparations that had been treated with NaOH, centrifuged to remove spores, restored to neutrality with HCl and filtered (0.45 micrometer filter) than in supernatants of whole culture preparations. Of the 28 Bt strains tested, eight [IBL 24, 136, 156, 194, 425, 455 (Dipel, commercial strain), 465 and 745] caused high levels of larval mortality after two-three days of feeding. Whole culture Bt preparations were more toxic than samples that contained spores alone, which, in turn, were more toxic than samples that contained primarily dissolved crystal protein. With the addition of any one of the three preparations of Bt, DBM mortality increased in a dose-dependent manner. Preparations of pH-treated Bt were not stable to boiling (therefore, not a heat-stable exotoxin) or, with the possible exception of Strain IBL 425, to extraction with methanol. When the various Bt strains were tested on Dipel-resistant DBM, IBL 425 had the lowest LC50, followed by 24 and 194. Thus, IBL 425 was the most promising for development as a bioinsecticide against DBM. Experiments are underway to characterize the toxic Bt-produced factor(s).