Ultrastructure of spermatozoon in soft-shelled turtle Trionyx sinensis was examined by transmission electron microscope. The mature spermatozoon is vermiform in shape, which consists of a head, connecting piece, middle piece, principal piece and end piece, similar in morphology to that of the turtles, Chrysemys picta and Chimemys reevesii. However, several structures are unique. In the head, acrosomal complex covers the anterior protrusion of the nucleus, where there is a narrow lucent space between the subacrosomal cone and the outer acrosomal cap. 3-5 intranuclear tubules go through the anterior tow-thirds of the nucleus and their central cores extend towards the perforatorial rods. The connecting piece is located in the concave implantation fossa in the caudal end of the nucleus. The mitochondrial sheath in the middle piece is composed of concentric mitochondria arranged in 7-8 circles each of which contains 5 mitochondria. Concentric layers in the mitochondrion are double-membranes and they are cristae mitochondriales actually. The concentric cristae mitochondriales and lots of glycogens show the relationship to the vitality maintaining during long-time storage of the spermatozoon in the turtle. The structural modal of proximal centriole is 9×3+0, in which the typical nine triplet microtubules arrange in a pinwheel fashion and are surrounded by a dense material. By comparison, the structural modal of distal centriole is 9×3+2, and the central microtubules and outer triplet microtubules are embed in dense fibers respectively. The distal centriole stands vertically to the proximal one. There is a wedge-shaped annulus around the end of the mitochondriall sheath. The axonemal complex of the principal piece is surrounded by several layers of circumferential fibers, which obviously produces a fibrous sheath reducing backward gradually in thickness. In the axonemal complex that posses a structural model of 9×2+2, there are two thick fibers corresponding to No.3 and No.8 doublet microtubules respectively, which me
