The reinvestigation of the fine structure of the mature spermatozoon of the proteocephalidean tapeworm Proteocephalus longicollis, a parasite of salmonid fish, has shown new characters not described previously for this species. They include a helicoidal crested body, 60-100 nm thick and localised at the anterior extremity of the spermatozoon. Also new is the finding that cortical microtubules in P. longicollis spermatozoon are of two types. The nucleus situated in the middle part of the gamete is rod-shaped anteriorly, but it is wound around one of the axonemes posteriorly. It is newly observed that both the anterior and posterior extremities of the spermatozoon of P. longicollis contain a single axoneme. The ultrastructural characteristics of the mature spermatozoon of P. longicollis coincide in their main features with those of the spermatozoon of P. torulosus, a parasite of cyprinid fish.