Jadwiga Wierzbicka and Ewa Sobecka
Agricultural University of Szczecin, Kazimierza Królewicza 4, 71-550 Szczecin, Poland

Key words: parasitic nematode, Hysterothylacium incurvum, morphology, fish, swordfish.

Abstract.
The alimentary tract (mainly the stomach) of a single swordfish (Xiphias gladius L.) caught in Georges Bank (NW Atlantic) yielded a total of 103 nematodes (72 females and 31 males) identified as Hysterothylacium incurvum (Rudolphi, 1819). A morphological description of the nematodes, complete with measurements of female and male specimens, and photographs are presented. We concluded that the labial and interlabial structures are the major characters which distinguish H. incurvum from H. corrugatum Deardorff and Overstreet, 1980. On the other hand, the ratio between spicule length and male body length cannot be used as a diagnostic character.