Perfringolysin O (PFO), a soluble toxin secreted by the pathogenic Clostridium perfringens, forms large homo-oligomeric pore complexes comprising up to 50 PFO molecules in cholesterol-containing membranes. In this study, electron microscopy (EM) and single-particle image analysis were used to reconstruct two-dimensional (2D) projection maps from images of oligomeric PFO prepore and pore complexes formed on cholesterol-rich lipid layers. The projection maps are characterized by an outer and an inner ring of density peaks. The outer rings of the prepore and pore complexes are very similar; however, the protein densities that make up the inner ring of the pore complex are more intense and discretely resolved than they are for the prepore complex. The change in inner-ring protein density is consistent with a mechanism in which the monomers within the prepore complex make a transition from a partially disordered state to a more ordered transmembrane beta-barrel in the pore complex. Finally, the orientation of the monomers within the oligomeric complexes was determined by visualization of streptavidin (SA) molecules bound to biotinylated cysteine-substituted residues predicted to face either the inner or outer surface of the oligomeric pore complex. This study provides an unprecedented view of the conversion of the PFO prepore to pore complex.