Relationship between hsp 70 expression and development of thermotolerance in the olive flounder was investigated by exposing the fish to 23 or 26$^{circ}C$ for 1 h. After selected intervals, they were re-exposed to 31$^{circ}C$ for a duration of 15 min. Theremotolerance rapidly developed with increasing interval and peaked at 3 h interval. Subsequently, it gradually decayed and disappeared by 24 h interval. The flounder displayed remarkably high levels of hsp 70 mRNA and protein, as compared to control flounder. Hence, the olive flounder acquires thermotolerance, which is positively correlated with the synthesis of hsp 70.