In previous study, scuticociliate P. dicentrachi was isolated from farmed flounder during the december 2004 to april 2005. Pathogenicity, mortality and infection symptom were studied using 3 cm and 5 cm groups of juvenile flounder. The ciliates were exponentially increased from bottom layer of the experimental tanks, which propagated within 2000 cells/ml (1.4 × 103 cell ㎖-1 ～ 2.5 × 103 cell ㎖-1) after 6 days of inoculation. The middle layer was maintained within 300 cells/ml. Both 3 cm and 5 cm groups were infected with ciliates, mainly 3 cm group showed high mortality during the experimental period. The death of 3 cm group was started from 5 days and showed 95.6% of mortality after 28 days of first inoculation. The control group showed 4.4% of mortality however, we could not observed any ciliates. The death of 5 cm group was started later than 3 cm group after 18 days of first inoculation. The total mortality was 71% during 28 days. No mortality and infection symptoms were observed in the control. We also studied SSU rRNA gene of ciliates which, re-isolated from infected flounder of experimental groups. When SSU rRNA in this study compared with previous data showed that the identified strain of both previous and present study was same.