Abstract: A capsalid monogenean, Pseudonitzschia uku Yamaguti, 1965, that belongs to Pseudonitzschia, Pseudonitzschiinae, was discovered on the gills of Aprion virescens in Qinglan Bay of Hainan in China. The morphology and size of the specimens were consistent with the original description, except that there were anchors and marginal hooklets on their haptor, and that the recurved spines over the cirrus were missing. Therefore here we updated the description of the characteristics of the Pseudonitzschiinae subfamily and the Pseudonitzschia genus. The subfamily, genus and species were reported for the first time in China. The specimens were deposited in the Lab of Fish Parasitology of Collage of Life Sciences in the South China Normal University. Amended diagnosis of Pseudonitzschiinae: Capsalidae sensu Yamaguti, 1963. Body elliptical flattened. Haptor disc-shaped, sessile, usually folded antero-posteriorly, armed with one pair of anchors and 14 marginal hooklets; indented muscular margin and delicate marginal membrane present; without septa, loculi or papillae. Anterior attachment organ paired, sucker-shaped. One pair of eye spots. Pharynx without constriction. Intestinal limbs with lateral dendritic diverticula, not united posteriorly. Testes numerous, intercecal. Male copulatory pouch muscular, postpharyngeal, consisted of ejaculatory and prostatic ducts; cirrus short, swelled anterior, armed or unarmed. Common genital pore opening ventrally, posterior to the left anterior attachment organ. Ovary single, spherical, pretesticular; Ootype well developed; uterus opened into the common genital pore; vagina opened dorsally at the level of pharynx. Vitellaria co-extensive with intestinal branches; vitelline reservoir immediately sinistral to ovary. Egg tetrahedral, with single filament. Gill parasites of marine teleosts.