Pore-forming toxins form a family of proteins that act as virulence factors of pathogenic bacteria， but similar proteins are found in all kingdoms of life， including the vertebrate immune system. They are secreted as soluble monomers that oligomerize on target membranes in the so-called prepore state; after activation， they insert into the membrane and adopt the pore state. Lysenin is a pore-forming toxin from the earthworm Eisenida foetida， of which both the soluble and membrane-inserted structures are solved. However， the activation and membrane-insertion mechanisms have remained elusive. Here， we used high-speed atomic force microscopy to directly visualize the membrane-insertion mechanism. Changing the environmental pH from pH 7.5 to below pH 6.0 favored membrane insertion. We detected a short α-helix in the soluble structure that comprised three glutamic acids (Glu92， Glu94， and Glu97) that we hypothesized may represent a pH-sensor (as in similar toxins， e.g.， Listeriolysin). Mutant lysenin still can form pores， but mutating these glutamic acids to glutamines rendered the toxin pH-insensitive. On the other hand， toxins in the pore state did not favor insertion of neighboring prepores; indeed， pore insertion breaks the hexagonal ordered domains of prepores and separates from neighboring molecules in the membrane. pH-dependent activation of toxins may represent a common feature of pore-forming toxins. High-speed atomic force microscopy with single-molecule resolution at high temporal resolution and the possibility of exchanging buffers during the experiments presents itself as a unique tool for the study of toxin-state conversion.