A bizarre new genus and three new species of parasitic Ichthyocephalidae are described and illustrated from the gut of Vietnamese diplopods. Ichthyocephaloides comatus sp. n., an atypical member of a predominantly Australasian genus, is characterised by the very posterior vulva (V = 92), single genital tract lacking a vaginal diverticulum, dorsal microtrichs long and curled but ventral microtrichs short and bristle-like, basal bulb barely wider than isthmus, short, conoid tail in both sexes, complement of 20 copulatory papillae, five pairs being precloacal and five pairs postcloacal, and spicule form and relative size. The extremely posterior vulva, lack of vaginal diverticulum, short, conoid tail in both sexes and presence of only 20 copulatory papillae immediately distinguish this species from I. dasyacanthus Hunt & Sutherland, 1984, I. xesmostoma Hunt, 1994, I. eurystoma Hunt, 1999 and I. longisaccus Hunt, 1999, all of which have a more anterior vulva, genital tract with a well developed vaginal diverticulum, a longer, subulate tail in both sexes and more numerous copulatory papillae. Xystrognathus gen. n., a highly unusual genus of ichthyocephalid, has an apparently primitive suite of characters and probably represents the oldest lineage of the family yet discovered. It is characterised within the Ichthyocephalidae by having 23 copulatory papillae, three of the postcloacal pairs being subdorsal or sublateral in position plus a single midventral papilla (well developed in the type, less so in the other species) on the anterior cloacal lip, a very posteriorly located vulva, digonant female genital system lacking a vaginal diverticulum, isomorphic and more or less isometric spicules and isthmus ca the same length or shorter than the height of the basal bulb. Xystrognathus gen. n. is the only genus known within the Ichthyocephalidae where the copulatory papillae are arranged in a pattern typical of the Afro-Asian Rhigonematoidea. It may represent the ancestral lineage of the family. Xystroganthus priscus gen. n., sp. n. and X. phrissus gen. n., sp. n. are described and differentiated from each other on the basis of female tail length and shape, development of the midventral papilla and form of the anterior vulval lip (deeply bilobed in the latter species).