Haploporus mugilis sp. n. (Digenea: Haploporidae) was obtained from the intestine of kandas, Valamugil engeli (Bleeker) (Mugilidae, Perciformes) in the Taiwan Straits, China. It most closely resembles Haploporus spinosus Machida, 1996 from the intestine of Crenimugil crenilabis in Japanese and adjacent waters in the intestinal bifurcation posterior to the acetabulum, in egg-size (0.039-0.044 × 0.019-0.022 in H. mugilis and 0.034-0.042 × 0.018-0.023 in H. spinosus) and in the hermaphroditic sac armed with spines. However, it differs from the latter in its hermaphroditic sac with two long and numerous small spines rather than with four long and six small spines and in the excretory vesicle with a single prominent refractory concretion instead of being empty.