Aim: This study was a part of an ongoing parasitological survey to investigate the health status of hill stream fish in river systems of Arunachal Pradesh - A global biodiversity hotspot.

Materials and Methods: During the 2013/2015, 18 live specimens of Barilius barna (Cyprinidae) were captured from the local rivers of Arunachal Pradesh and examined for parasitic monogenoids. These fish, with their flatworms, were immediately fixed in hot (60°C) 4% formalin for later examination. Identification and morphometric description used in this study followed Gussev (1976). Type specimens were deposited in the British Natural History Museum, UK.

Results: Dactylogyrus barnae sp. n. is described and illustrated from specimens of B. barna (Hamilton, 1822) from Arunachal Pradesh, India. The new species is characterized by a combination of the following characters: Copulatory tube coiled in 1½ counterclockwise rings, vagina consisting of a vaginal tube and vaginal pore, a complex sclerotized plate of unknown function in between male copulatory organ and vagina, and an anteromedial knob-like process on the dorsal bar.

Conclusion: D. barnae sp. n. is the fourth species of Dactylogyrus described from the Northeast India and brings the total number of species of Dactylogyrus in Indian waters to 56. B. barna represents a new host record for Dactylogyrus spp., and possibly the first report for any parasite.

Keywords: Arunachal Pradesh, Barilius barna, Dactylogyrus barnae sp. n., monogenoidea, sclerotized plate.

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