The insecticidal delta endotoxin of Bacillus thuringiensis was labeled with iodine-125. Brush-border membrane vesicles, prepared from the midgut epithelium of Pieris brassicae larvae, known to be highly susceptible to the toxin, and from a non-target tissue: the small intestine of rat, were examined for binding of 125I-toxin. The toxin was bound specifically only to insect vesicles. Its binding to the insect membrane system was competitively inhibited by 127I-toxin and non-iodinated toxin, whereas the binding of the 125I-toxin to the mammalian membrane system was not affected by unlabeled toxin. Vesicles of P. brassicae possess two individual binding-site populations for iodinated toxin with dissociation constants of 46 nM and 490 nM. The Hill coefficients of both sites were approximately 1 and the binding capacities were 0.2 pmol and 30 pmol/mg vesicle protein for the high and the low-affinity sites respectively. The estimation of the dissociation constant for non-iodinated toxin, using a competition experiment, revealed only one binding-site population which possessed a dissociation constant of 235 nM. It is concluded that this is the binding site for the native toxin. This site was sensitive towards treatment with proteases or mixed glycosidases. It is suggested that it is a protein or a glycoprotein.