Abstract Helical, non-cellulosic microfibrils are formed in water from substance(s) soluble in 80% ethanol, which are concentrated in (or close to) the cell walls of Acetobacter xylinum and Acetobacter suboxydans but not in those of Acetobacter acetigenum or Acetobacter aceti . The microfibrils, which are variable in diameter and pitch, aggregate readily to form bundles. They appear to be formed spontaneously by packing of non-polymeric material to form extended threads, without formation of covalent bonds. Formation of the helices may be prevented or destroyed by the addition of either oxidizing or reducing agents to the ethanol extracts. The microfibrils do not appear as such in the bacterial cell wall. The monomeric unit of the microfibrils may be a long-chain aliphatic ester or ketone with hydroxyl groups attached.