1. 1. The swimming crab, Ovalipes punctatus , was used during the summer and winter months of 1980 for respiratory studies. 2. 2. The oxygen consumption showed that the crabs had a circadian rhythm with a periodicity of ca . 24 hr, the crabs being mainly active at night (up to eight times higher then the value recorded during the day). 3. 3. The mean oxygen consumption for O. punctatus (60 g live mass: 121 μg O 2 /g per hr; 15 g live mass: 195 μg O 2 /g per hr) agreed favourably with values quoted for the Brachyura. 4. 4. The Q 10 values range from 1.61 to 4.50 and were dependent on temperature (at 15, 20 and 25°C) but not on body mass. 5. 5. During starvation there was a marked decline in the oxygen consumption of the crabs, which stabilized after 10–12 days.