Abstract: Sperm ultrastructure of the freshwater turtle Mauremys caspica (M. caspica) has not yet been published. Therefore, the present study was carried out to elucidate the ultrastructural details of the sperm of this species. The spermatozoa of M. caspica are filiform in shape with a curved head measuring 7.9 μm. Acrosome complex (acrosomal cap and subacrosomal material) measures 3.7 μm, nucleus is 6.5 μm (2.3 μm acrosomal region, 4.2 μm post-acrosomal region) in length and midpiece and tail (principal and end pieces) are maximally of 4.2 μm and 35.8-38.6 μm length, successively. Diameter of the different tail pieces is gradually diminished from midpiece (1.2 μm) toward principal (0.5 μm) and end (0.3 μm) pieces. The anterior tapered nuclear tip is caped with the conical sheath formed by the acrosome and the subacrosomal material. Endonuclear canals (2-3) extend from the nuclear rostrum deep into the nucleus. Midpiece encompasses a mitochondrial sheath (total of 40 mitochondria) encircling the axonemal complex but lacks a fibrous sheath. Mitochondria are rounded having concentric cristae and central dense bodies. Annulus is present as a dense ring marking the distal end of midpiece. The principal tail piece consists of a typical axoneme (9+2 pattern of microtubular arrangement) surrounded by the fibrous sheath and plasma membrane. The axoneme is surrounded only by the plasma membrane in the end piece which lacks the fibrous sheath. The present findings were discussed and compared with that reported in other reptilian species.