During the years 2004 and 2005 high mortalities in turbot Scophthalmus maximus (L.) from a fish farm in the north of Portugal were observed. Moribund fish showed darkening of the ventral skin, reddening of the fin bases and distended abdominal cavities caused by the accumulation of ascitic fluid. Ciliates were detected in fresh mounts from skin, gill and ascitic fluid. Histological examination revealed hyperplasia and necrosis of the gills, epidermis, dermis and muscular tissue. An inflammatory response was never observed. The ciliates were not identified to species level, but the morphological characteristics revealed by light and electronic scanning microscopes indicated that these ciliates belonged to the order Philasterida. To our knowledge this is the first report of the occurrence of epizootic disease outbreaks caused by scuticociliates in marine fish farms in Portugal.