Ligophorus llewellyni n. sp. (Ancyrocephalidae: Ligophorus Euzet & Suriano, 1977) is described from the gills of Liza haematocheilus (Temminck & Schlegel) introduced into the Black Sea from the Far East. Ligophorus llewellyni is closely related to L. pilengas Sarabeev & Balbuena, 2004, which parasitises the same host species. The two species differ in the morphology of the accessory piece of the copulatory organ and in some of the characters of the haptoral hard-parts. The morphometric variability of L. llewellyni and in its morphologically most similar congeners from the Black Sea is studied. Correlations between 30 morphometric characters of the haptoral hard-parts and the significance of each for species differentiation are examined. It is suggested that only 22 characters are useful as diagnostic criteria permitting the differentiation of morphologically similar species of Ligophorus.