This Study describes columnaris disease from cultured rainbow trout (Oncorhynchus mykiss) fry in Turkey in July, 2004. The infection appeared in rainbow trout fry between 5 and 10 g weight in reared raceways where mortality level reached approximately 30% in a day. The appearance of sick fish, besides characteristic skin discolouration and yellow necrotic areas was seen at the tip of the gills. Grain negative long, thin rods were isolated front the anterior kidney, liver and gills of sick fish. Flavobacterium columnare was identified by morphology, physiology and biochemical testing using conventional methods and the API 20 E test system.