Tristomella laevis (Monogenea. Capsalidae) was recorded on gills of sword-fish (Xiphias gladius), collected from Ayvacik, Canakkale, for the first time in Turkey. Infested fish weight ranged between 8.1 and 10.5 kg. Parasites were observed generally on small fish rather than big fish. Main prevalence of parasites was determined as 70% and serious gill damage was observed on the host fish.