The rates of growth and oxygen consumption in vitro of Ruditapes philippinarum were examined in relation to their high temperature tolerance. Two groups of the clam (16.7 and 11.4mm mean shell length) were fed cultured algae Pavlova lutheri for 4 weeks under various constant temperatures between 10 and 34°C. Cumulative mortality in each group during the experiment was 0 to 3% except at 34°C. All clams reared at 34°C died within 10 days. The growth rate (daily percent increase in shell length) was maximum at about 25°C. A condition index (dry weight ratio of shell to soft body) declined at 30°C. The oxygen consumption rate was determined volumetrically at intervals of 5°C from 5 to 50°C using specimens of 31.1mm mean shell length. The oxygen consumption rate decreased markedly above 40°C. Q10 for the oxygen consumption rate between 5 and 25°C remained close to 2.0, but decreased from the value above 25°C. The temperature considered to be optimum for the oxygen consumption was coincident with the maximum growth temperature. These results suggest that Ruditapes philippinarum probably suffers from thermal stress over 25°C, and has significant mortality at around 34°C within a few days and no heat resistance over 40°C.