Abstract
An optical diffraction study has been made of electron micrographs of extended sheaths of pyocin R. It has been demonstrated that the extended sheath consists of annuli of six subunits, which are arranged in a helix of 3.57 annuli per one turn. The annulus repeat in the helix direction is 35 A. The dimensions and probably the helical parameters of pyocin R sheath are different from those of T4 phage tail, but a structural correlation appears to exist between the extended sheath of pyocin R and that of the phage tail.
