The helminth fauna of the gastrointestinal tract and abdominal cavity of cutlass fish, Trichiurus lepturus L., off the Taiwanese coast of the north-western Pacific was investigated. The following helminths were found: (1) nematodes--Anisakis simplex, Hysterothylacium aduncum, Porrocaecum decipiens, Raphidascaris trichiuri; (2) digeneans--adult Lecithochirium trichiuri; and (3) cestodes-plerocercoids of Proteocephalus spp. The third-stage larvae of these four anisakid nematodes were characterized genetically using a molecular approach. The nuclear ribosomal DNA region spanning the first internal transcribed spacer (ITS-1), the 5.8S gene and the second internal transcribed spacer (ITS-2) was amplified and sequenced. Based on the sequence differences, a PCR-based restriction fragment length polymorphism method was established for the unequivocal delineation of the four species. Phylogenetic analysis showed that H. aduncum clustered with P. decipiens, whereas A. simplex was not closely related to these according to the nucleotide sequences of all rDNA.