The binding component (Vip1Ac) of the ADP-ribosylating vegetative insecticidal protein (Vip) of Bacillus thuringiensis HD201 was isolated from the supernatant of cell cultures. Vip1Ac protein solubilized at room temperature ran as oligomers on SDS-PAGE. These oligomers were not resistant to heating. Mass spectroscopic analysis of this high molecular mass band identified it as Vip1Ac. The protein formed in artificial lipid bilayer membranes channels with two conductance states of about 350 and 700 pS in 1 M KCl. The channel conductance showed a linear dependence on the bulk aqueous KCl concentration, which indicated that the channel properties were more general than specific. Zero-current membrane potential measurements showed that the Vip1Ac channel has a slightly higher permeability for chloride than for potassium ions. Asymmetric addition of Vip1Ac to lipid bilayer membranes resulted in an asymmetric voltage dependence, indicating its full orientation within the membrane. The functional role of Vip1Ac and its relationship to other ADP-ribosylating toxins are discussed.