Rapid cryo-fixation of mouse spermatozoa followed by freeze substitution, thin section and freeze-fracture demonstrates the finer detail possible with quick-freeze as compared to chemical fixation. As seen in thin sections the unit membrane is composed of 2-4 nm size particles and the cytoplasm, mitochondria, and axonemal filament components all appear rich in fine structure detail. The filamentous structure of the post-acrosomal sheath and its connection with the plasmalemma is presented and compared to previous studies on this structure. Freeze-fracture data demonstrates 7-9 nm size, plasmalemmal, PF-face particles most heavily concentrated in the region just ahead of the striated ring. The outer acrosomal EF-face contains linear arrays of 7-9 nm size EF-face particles. The inner acrosomal membrane contains scattered, 7-9 nm size PF-face particles. The inner and outer nuclear membranes also contain scattered, 7-9 nm size particles. The results of this study present data which supports and extends previous studies on mouse spermatozoa. The results are discussed in terms of the advantages of cryo-fixation and freeze-substitution compared to conventional fixation in the preservation of fine structure detail.