Hysterothylacium aduncum (Nematoda: Anisakidae) infecting a herbivorous fish, Siganus fuscescens, off the Taiwanese coast of the Northwest Pacific. Zoological Studies 41(2): 207-214. Hysterothylacium aduncum (Rudolphi, 1802) infecting a herbivorous fish, Siganus fuscescens, and 1 invertebrate zooplankton, a chaetognath, is reported off the Taiwanese coast of the Northwest Pacific. Third-stage larvae were found in the body coelom of the chaetognath, Sagitta sp. Fourth-stage larvae and adults were recovered from the stomach and intestines of S. fuscescens. Nematodes are described, measured, and illustrated. The prevalence of parasitic infection, mean intensity, and abundance were calculated from samples collected from 2 different Taiwanese coastal locations. The life cycle of H. aduncum, involving the host species considered, is also postulated.