The ultrastructure of spermiogenesis and the spermatozoon of Poracanthium furcatum (Opecoelidae), parasite of the fish Mullus surmuletus, were studied by electron microscopy. Results were compared with those documented for other digeneans, in particular with Opecoeloides furcatus, another opecoelid. Spermiogenesis follows the general pattern found in the Digenea. Nevertheless, a novel ultrastructural element is described. The posterior part of the centriole is unusual in that it comprises a central element. The mature spermatozoon of P. furcatum presents some characteristics allowing distinction between it and O. furcatus, contrary to the external morphology. It contains a lateral expansion, two mitochondria and a nuclear biflagellar region. Other important ultrastructural features in the spermatozoon include: external ornamentations of the plasma membrane, spine-like bodies, anterior and posterior extremities. All these elements of the male gamete are interesting criteria for phylogenetic studies.