By using transmission electron microscopy (TEM) the events of spermatogenesis are described for the first time in the tuatara Sphenodon punctatus punctatus (Gray), a representative of the `reptilian' order Sphenodontida. Secondary spermatocytes contain two greatly elongate (8.0 <latex>$\mu $</latex>m), rod-shaped centrioles which lie parallel to one another and are each associated with a small deposit of dense material and a short centriole. Spermatids contain only one rod-shaped centriole (associated with a short centriole) which gives rise to the flagellar axoneme thereby becoming the distal centriole. Four stages of spermatid development can be distinguished: (i) the early stage (nucleus round; nuclear contents granular with a thin, condensed periphery; mitochondria scattered; acrosomal vesicle spheroidal, slightly depressed onto nuclear surface); (ii) the middle stage (nucleus pyriform with two endonuclear canals formed; nuclear contents fibro-granular with thick periphery; mitochondria chiefly posterior; acrosomal vesicle flattened; centriolar complex attached to nucleus); (iii) the advanced stage (nucleus elongate and rod shaped; nuclear contents coarsely granular; mitochondria (containing linear cristae) clustered around the distal centriole; acrosomal vesicle conical; centriolar complex attached to posterior fossa of nucleus); (iv) the late stage (nucleus very elongate and associated with a longitudinally arranged microtubular sheath; nuclear contents very condensed; midpiece fully formed and featuring mitochondria with concentric cristae and a dense intramitochondrial body; centrioles associated with a dense, lateral body). Testicular sperm have a conical acrosomal vesicle (length 4 <latex>$\mu $</latex>m) and subacrosomal cone, an elongate (length 54-56 <latex>$\mu $</latex>m) helical nucleus, a midpiece (length 8 <latex>$\mu $</latex>m, featuring spheroidal mitochondria containing concentric cristae and a dense body), an annulus, an elongate principal piece (length 74-78 <latex>$\mu $</latex>m, featuring a dense, fibrous sheath) and a short end piece (length 2-4 <latex>$\mu $</latex>m). Epididymal sperm differ from those in the testis by having a more developed lateral body in the midpiece and a sheath of flocculent material surrounding the fibrous sheath in the principal piece. The relatively large number of epididymal sperm still associated with a cytoplasmic droplet suggests that sperm spend a significant period maturing within the epididymis. The features of spermatogenesis and mature sperm suggest that the Sphenodontida are primitive amniotes, with only chelonians having fewer spermatozoal apomorphies while the crocodilians are little more advanced.