To examine the binding of Bacillus thuringiensis ␦-endotoxins, CryIAa, CryIAb, and CryIAc, to Lymantria dispar (gypsy moth) brush border membrane vesicles (BBMV), saturation kinetic analyses were conducted according to a two-step interaction schemefor ␦-endotoxin binding to BBMV, rather than the onestep reversible binding presented in prior reports.The order of toxicity of the ␦-endotoxins, as measured by the dose required for a 50% inhibition of weight gain (ID 50 ), was CryIAa (77.3 ng) > CryIAb (157 ng) > CryIAc (187 ng). While both the maximum extent of binding, B max , and the half-maximum insertion rate concentration, K1 ⁄2 , was observed to be indirectly related to toxicity, the rate constant of irreversible binding, k 2 , was found to be directly correlated to toxicity.