Boar spermatozoa were observed by a rapid-freeze, deep-etch, and rotary shadowing technique. Domains of particles were found on the ES face of the plasma membrane in the proximal part of the postacrosomal region. Particles are 17 nm in diameter, have a central pit, and appear to contain either five or six subunits. The diameter of the subunits is about 7 nm. The particles are arranged hexagonally within the domain. Straight parallel ridges, 15 nm wide, were found on the entire PF face of the plasma membrane in the postacrosomal region. The ridges run almost parallel to the long axis of the spermatozoon. Part of the postacrosomal sheath could be seen through artificial openings in the PF face of the plasma membrane. The postacrosomal sheath seems to be a sheet of closely packed particles 7 nm in diameter and has striations with a periodicity of 15 nm. The ridges on the PF face of the cell membrane appear to be reflected by the striations of the postacrosomal sheath. In the neck and tail regions of the spermatozoa, orderly arranged particles were observed on the ES and PF faces of the plasma membrane.