Clostridium perfringens perfringolysin O (PFO or Θ-toxin) is a cytolytic toxin that binds to cholesterol-containing membranes and then self- associates to spontaneously form aqueous pores of varying size in the bilayer. In this study, a membrane-spanning domain has been identified in PFO by a combination of fluorescence spectroscopic methods using the fluorescent dye N,N'-dimethyl-N-(iodoacetyl)-N'-(7-nitrobenz-2-oxa-1,3- diazolyl)ethylenediamine (NBD) whose emission properties are sensitive to water. PFO was substituted with a single cysteine at most of the residues between amino acids K189 and N218, and then each cysteine was modified with NBD. Each purified NBD-labeled PFO was then bound to membranes, and the probe's environment was ascertained by measuring its fluorescence lifetime, emission intensity, and collisional quenching with either aqueous (iodide ions) or nonaqueous (nitroxide-labeled phospholipids) quenchers. Lifetime and intensity measurements revealed that the amino acid side chains in this region of the membrane-bound PFO polypeptide alternated between being in an aqueous or a nonaqueous environment. This pattern indicates that this portion of the membrane-bound PFO spans the membrane in an antiparallel β-sheet conformation. The alternating exposure of these residues to the hydrophobic interior of the bilayer was demonstrated by their susceptibility to quenching by nitroxide moieties attached to phospholipid acyl chains. Residues K189- N218 therefore form a two-stranded, amphipathic β-sheet in the membrane- bound PFO that creates a stable interface between the pore and the membrane. This same region packs as three short α-helices in the soluble, monomeric form of PFO, and therefore, the cholesterol-dependent conversion of PFO to a membrane-bound oligomer involves a major structural transition in which three α-helices unfold to form a membrane-spanning amphipathic β-sheet.