The present paper describes the ultrastructure of spermatozoa ofTaenia mustelaeas revealed by transmission electron microscopy. The mature spermatozoon of this tenid is filiform, tapered at both ends, and lacks mitochondrion. It is capped by an apical cone of electron-dense material and presents a single helical crest-like body 75-nm thick. The axoneme is of the 9+‘1’ pattern of trepaxonematan platyhelminthes and lacks a periaxonemal sheath. Transverse intracytoplasmic walls are observed in several regions of the spermatozoon. In cross-section, the spiralled nucleus shows a horse-shoe to annulus shape. The submembranous cortical microtubules are spiralled at an angle of about 45° to the hypothetical spermatozoon axis.