-Barrel pore-forming toxins ( -PFT), a large family of bac- terial toxins, are generally secreted as water-soluble monomers and can form oligomeric pores in membranes following proteo- lytic cleavage and interaction with cell surface receptors. Mona- lysin has been recently identified as a -PFT that contributes to the virulence of Pseudomonas entomophila against Drosophila. It is secreted as a pro-protein that becomes active upon cleavage. Here we report the crystal and cryo-electron microscopy struc- ture of the pro-form of Monalysin as well as the crystal struc- tures of the cleaved form and of an inactive mutant lacking the membrane-spanning region. The overall structure of Monalysin displays an elongated shape, which resembles those of -pore- forming toxins, such as Aerolysin, but is devoid of a receptor- binding domain. X-ray crystallography, cryo-electron micros- copy, and light-scattering studies show that pro-Monalysin forms a stable doughnut-like 18-mer complex composed of two disk-shaped nonamers held together by N-terminal swapping of the pro-peptides. This observation is in contrast with the mono- meric pro-form of the other -PFTs that are receptor-depen- dent for membrane interaction. The membrane-spanning region of pro-Monalysin is fully buried in the center of the doughnut, suggesting that upon cleavage of pro-peptides, the two disk-shaped nonamers can, and have to, dissociate to leave the transmembrane segments free to deploy and lead to pore formation. In contrast with other toxins, the delivery of 18 sub- units at once, nearby the cell surface, may be used to bypass the requirement of receptor-dependent concentration to reach the threshold for oligomerization into the pore-forming complex.