Scuticociliates Miamiensis avidus (syn. Philasterides dicentrarchi) causes high mortality and bad growth in olive flounder Paralichthys olivaceus. Temperature is an important factor not only for growth of pathogens but also for host immune system in poikilothermal animal. In this study, temperature affecting ciliate growth and pathogenicity against olive flounder were examined. Doubling time for the ciliate growth was 61.82 hours at $5{^{circ}C}$, 26.32 hours at $10{^{circ}C}$, 21.14 hours at $15{^{circ}C}$, 16.86 hours $20{^{circ}C}$ and 16.21 hours at $25{^{circ}C}$. Maximum ciliate numbers were similar at $10-20{^{circ}C}$ at the range of $1.54-1.75{ imes}10^{5}$/ ml. Duplicated intraperitoneal injections were conducted with the ciliates by the concentrations of $1{ imes}10^{2}$, $1{ imes}10^{6}$, $1{ imes}10^{4}$and $1{ imes}10^{5}$/ fish (average 8.34 cm, 4.33 g) then kept at $10{^{circ}C}$, $15{^{circ}C}$ and $20{^{circ}C}$. Cumulative mortality was low at $10{^{circ}C}$ and the mortality was increasing at higher water temperatures. In addition, cumulative mortality was higher at higher dose of infections. In conclusion, Scuticocilite M. avidus grew well at higher temperature (at $5{^{circ}C}$, $10{^{circ}C}$, $15{^{circ}C}$ and $25{^{circ}C}$) in vitro, and olive flounder mortality due to M. avidus was highly water temperature and dose dependent. The results of this study suggest that water temperature control may one of the essential factor to reduce mortality due to M. avidus infection.