Allopseudaxine katsuwonis is a species of monogenean flatworm, which is parasitic on the gills of a marine fish.

[2] Two years later, Ishii & Sawada gave an English description of this species.

[1] Subsequently, Price redescribed this species, and pointed the presence of the vaginal "gill-like" openings.

The digestive organs include an anterior mouth, an oval pharynx, a simple oesophagus and a posterior intestine that bifurcates at or near level of genital pore in two lateral branches.

The reproductive organs include an anterior genital atrium armed with a corona of 14 hook-like spines, one row of dorsolateral marginal vaginae on each side of the body, each row divided into a longitudinal row of transverse slit with sclerotized margins, a slender tubular ovary and about 60 large oval testes pre, para and post-ovarian.