The mitochondrial sheath is composed of mitochondria that coil tightly around the midpiece of the sperm flagellum. Mitochondria are recruited from the cytoplasm to the flagellum late in spermatogenesis. Recruited mitochondria are initially spherical, but then elongate laterally to become crescent-like in shape. Subsequently, these crescent-like mitochondria elongate continuously to coil tightly around the flagellum. Mitochondrial sheath development in glycerol kinase 2 (Gk2)-disrupted mice, which show abnormal mitochondrial sheath formation, was observed using freeze-fracturing coupled with scanning electron microscopy (Shimada et al., Glycerol kinase 2 is essential for proper arrangement of crescent-like mitochondria to form the mitochondrial sheath during mouse spermatogenesis, pp. 155–162). Gk2-disrupted spermatids show abnormal localization of crescent-like mitochondria, despite initially exhibiting proper alignment of spherical mitochondria around the flagellum. These results indicate that GK2 is essential for proper arrangement of crescent-like mitochondria during mitochondrial sheath formation in mouse spermatogenesis.