SUMMARY Pyocin sheaths treated with sodium dodecyl sulphate (SDS) assembled into long rod structures in saline phosphate buffer but not in tris-HCl buffer. For the effective formation of long rods, 0.05 M or more NaCl was required in the reaction mixture and the optimal concentration of SDS was 0.05 to 0-2 yo. Morphological observation revealed that each long rod was formed by aggregation of several single sheaths. The mechanism of long rod formation by SDS is discussed in relation to the interaction of soluble proteins with SDS.