The prevalence of the scuticociliatosis in flounder farms of Jeju island was surveyed for ten years from 1995 to 2004. The occurrence ratio had maintained as less than 10% until mid 1990s, and shown increasing trends to year 1998. The ratio was equivalent to over 40% of overall disease occurrences in flounder farms since year 2000. The monthly infection rates by the scuticociliates indicated relatively higher levels from May to September, and mixed infection phenomena with bacterial disease of vibriosis were common.