Pale chubs were collected from several localities during the period from 1992 through 1997. Living worms were fixed in hot AFA (ethanol-formalin-acetic acid), stored in 70% ethanol, and stained with acetocarmine in the routine preparation of whole mounts. Specimens were measured with an ocular micrometer, and were drawn with the aid of a camera lucida. Measurements, unless otherwise stated, are in millimeters.

The description is based on 5 mature specimens. Each value is the mean with the range in parentheses. Body elongated oval. 1.63 (1.45-1.84) long by 0.82 (0.65-0.94) in maximum width. Tegument smooth. Oral sucker subterminal, 0.22 (0.19-0.28) long by 0.24 (0.20-0.32) wide. Ventral sucker pre-equatorial. 0.35 (0.27-0.41) long by 0.35 (0.28-0.40) wide. Ratio of mean diameters of oral and ventral suckers 1:1.54 (1:1.35-1.65). Prepharynx short. Pharynx well developed, 0.12 (0.10-0.15) in both length and width. Intestinal caeca simple, extending to the level of the posterior border of posterior testis.

Testes tandem, in posterior intercaecal region, irregular in shape; anterior testis 0.19 (0.10-0.23) long by 0.22 (0.13-0.30) wide; posterior testis 0.23 (0.16-0.29) long by 0.24 (0.14-0.31) wide. Cirrus sac elongated clavi form, overlapping dorsally anterior margin of ventral sucker. Seminal vesicle bipartited by a constriction; twisted, large proximal region and oval, small distal region. Pars prostatica tubular, surrounded by well developed prostatic cells. Ejaculatory duct short, joining metraterm to form shallow genital atrium. Genital pore ventral, sinistral, at level of pharynx.

Ovary deeply bilobed or trilobed, pretesticular or at right of anterior testis, 0.15 (0.09-0.18) long by 0.13 (0.09-0.16) wide. Seminal receptacle dorsal, round or oval, just left to ovary or overlapping anterior testis, 0.11 (0.07-0.14) long by 0.12 (0.08-0.18) wide. Oviduct passing antero-laterally from ovary, receiving duct from seminal receptacle and common vitelline duct before passing into Mehlis' gland. Laurer's canal passing transversely to left, opening dorsally. Ootype complex anterior to seminal receptacle. Vitelline follicles extending from posterior or lateral margin of pharynx to posterior border of posterior testis, embracing caeca, absent from peripheral fields of body. Anterior portion of left vitelline follicles being obstructed by cirrus sac. Uterus extending to posterior border of anterior testis. Eggs oval; 71 µm (65-75 µm) long by 40 µm (37-41 µm) wide.

The genus Neoplagioporus was newly proposed by Shimazu (1990) to accomodate three species, N. zacconis (Yamaguti, 1934), N. ayu (Takahashi, 1928) and N. elongatus (Goto et Ozaki, 1930). He distinguished it from other genera of the subfamily Plagioporinae by a distinctly bipartite seminal vesicle, submedian sinistrally situated genital pore, trilobated or bilobated ovary, and extensively distributed vitelline follicles. The closely related genera - Podocotyle, Plagioporus and Caudotestes - are distinguished from Neoplagioporus by above-mentioned characters. Therefore, we accepted the new genus Neoplagioporus proposed by Shimazu (1990).

The morphological characteristics of the present specimens well corresponded to those of the genus Neoplagioporus. In species level, however, the present specimens were different in the ovary shape from that described by Yamaguti (1934) and Shimazu (1990). The shape of the ovary was not trilobated but bilobated except one specimen among examined 10 specimens, and the direction of splitting of the ovary was the lateral face of it not the posterior. The other characteristics of the present specimens well coincided with those of N. zacconis (Table 1). Therefore, we considered the difference of the ovary shape as a character of geographical phenotypic variation, and proposed that the character state "bilobed or trilobed ovary" should be included in the description of N. zacconis.

The recorded hosts of N. zacconis are Zacco temmincki, Z. platypus, Oncorhynchus masou in Japan (Yamaguti, 1934 & 1942; Shimazu, 1990), and Opsariichthys bidens in China (Wang et al., 1985). In Korea, Z. platypus is a definitive host for N. zacconis.

Neoplagioporus zacconis differs from the other two species - N. ayu and N. elongatus - by having broader oval body shape and the confined distribution of vitelline follicles which are absent from the peripheral region of the body.