Ultrastrncture 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. How-ever, several structures are uniqne. In the head, acrosomal complex covers the antelior protrusion of the nnchius, where there is a narrow lucent space between the subacrosonlal cone and the outer acrosomal cap. 3-5 intrannclear tubules go through the antelior tow-thirds of the nnchius and their central cores extend towards the perforatorial rods. The conect-ing piece is located in the concave implantation fossa in the caudal end of the nnchius. The mitochonchial sheath in the middle piece is composed of concentric mitochonchia arranged in 7-8 drdes each of which contains 5 mitochonchia. Con-centric layers in the mitochonchion are double-membranes and they are cristae mitochonchiales 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 tinfie. The sturctural modal of proximal centriole is 9×3+0, in which the typical nine tliplet microtubules arrange in a pinwheel fashion and are surrounded by a dense material. By comparison, the structural modal of distal centliole is 9×3+2, and the central microtnbldes and outer triplet microtnbules are embed in dense fibers respec-tively. The distal centliohi stands vertically to the proximal one. There is a wedge-shaped anmdus around the end of the mitochonhriall 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 modol of 9×2+2, there are two thick fibers corresponding to No.3 and No.8 doublet microtubules re-pectively, which merge with the fibrous sheath. On the transection of principal piece, two dynein arms attached to A mi-crotnbble of the doublet which connected with the central microtnbules ttn-ongh an obvious radial spoke. Two central mi-crotnbbles are hnked by a bridging substance. The end piece tufty contains an axonemal complex surroundod by cell mem-brane, the microtubules in which are dispersed gradually. The structural characteristics above were different evidently from other class animals or other ordor animals of the reptile