A bacteriocin produced by strain 100052 of Shigella sonnei (shigellacin 52) was purified about 80-fold from a mitomycin C-induced culture supernatant. The purified preparation gave a single band on sodium dodecyl sulfate-gel electrophoresis with an approximate molecular weight of 10,000. The negatively stained electron micrograph of the purified bacteriocin preparation revealed the existence of three different particles. They were a small cylindrical tube (4.2 by 6.0 nm), a ring-shaped particle and a filamentous structure. The molecular weight calculated from the shape of the small tube was approximately 50,000. The latter two particles appeared to be constructed from subunits which were morphologically similar to the cylindrical tube. These results suggested that the morphological subunit of shigellacin 52 consisted of five chemical subunits with molecular weights of 10,000 each, and it assembled into two types of polymers, a ring-shaped particle and a filamentous structure, which seemed to be the main components of the purified shigellacin 52 preparation.