We report the presence of the parasitic copepod Ergasilus mosulensis Rahemo, 1982 (Copepoda, Ergasilidae) on the Caspian sheaya, Chalcalburnus mossulensis (Heckel, 1843) (Teleostei, Cyprinidae), caught from Atatürk Dam Lake (36° 41' N 37° 49' E, Sanliurfa, Turkey) between July and August 2006. A total of 23 Ergasilus mosulensis Rahemo, 1982 (Ergasilidae) were recorded from the gills of this fish species for the first time. Percentage of prevalence and mean intensity of infection was 10.8 and 1.8, respectively. The morphological characteristics of Ergasilus mosulensis were studied by means of scanning electron microscopy
