Cholesterol-dependent cytolysins such as perfringolysin O (PFO) are proteins that lyse cells by forming large pores in the target cell membrane; they contribute to infections ranging from food poisoning to pneumonia and listeriosis. PFO monomers bind the target membrane and then oligomerize to form pores of 20-50 subunits, via a pre-pore intermediate. Here we study the assembly of these pores using single-molecule fluorescence imaging in Droplet Interface Bilayers. We track the increase in brightness as labelled monomers assemble to form individual pores, and observe intermediate stoichiometries during the assembly process.