A new, host specific digenetic trematode, Neogonapodasmius hemirhamphi n. gen., n.sp. from the half beak, Hemirhamphus xanthopterus (Val.) off the Southwest (Trivandrum) coast of India is described. The parasite is found encysted on the anterior dorsal fin of the fish. Each cyst usually contains one female worm. Occasionally, however, one male also occurs along with the female, in a cyst. A comparison of the characters of the known forms of the family Didymozoidae with those of the present specimen showed significant differences prompting the erection of a new genus Neogonapodasmius, to contain the new specimen.