Ultrastructural observations on mature sperm ofPomphorhynchus laevis(Müller, 1776), the dominant acanthocephalan parasite of fish from the River Brenta, were carried out by scanning (SEM) and transmission electron microscopy Adult males ofP. laeviswere obtained from the digestive tract ofLeucisctcs cephalus(Pisces: Cyprinidae). The spermatozoon ofP. laevisis inverted with respect to conventional sperm models; it is filiform and consists of an anterior flagellar apparatus and a posterior nucleocytoplasmic derivative. The latter includes a layer of condensed chromatin beneath the plasma membrane opposite to the flagellum, numerous large subspherical electron dense inclusions, glycogen deposits and a pentalaminar structure next to the flagellum. The nucleocytoplasmic derivative terminates caudally with a flat lamina devoid of the dense inclusions and glycogen deposits. Proceeding through the posterior lamina, the 9+2 axoneme progressively disorganizes. The ultrastructural organization resembles that reported in other acanthocephalan species, although the number of central axonemal microtubules does not vary. Hence the atypical9+naxonemal pattern with O<n<5 is still only found in the eoacanthocephalan sperm.