Oxygen consumption (Mo2), heartbeat rate and form, and circulating hemolymph oxygen content were measured in relation to temperature in the large Antarctic infaunal bivalve Laternula elliptica. After elevations in temperature from 0° to 3°, 6°, and then 9°C, Mo2 and heartbeat rate rose to new levels, whereas maximum circulating hemolymph oxygen content fell. At 0°C, Mo2 was 19.6 μmol O2 h-1 for a standard animal of 2-g tissue ash-free dry mass, which equates to a 8.95-g tissue dry-mass or 58.4-g tissue wet-mass animal. Elevation of metabolism following temperature change had acute Q10 values between 4.1 and 5, whereas acclimated figures declined from 3.4 (between 0° and 3°C) to 2.2 (3°-6°C) and 1.9 (6°-9°C). Heartbeat rate showed no acclimation following temperature elevations, with Q10 values of 3.9, 3.2, and 4.3, respectively. Circulating hemolymph oxygen content declined from 0° to 3° and 6°C but stayed at a constant Po2 (73-78 mmHg) and constant proportion (∼50%) of the oxygen content of the ambient water. At 9°C, Mo2 and heartbeat rate both peaked at values 3.3 times those measured at 0°C, which may indicate aerobic scope in this species. After these peaks, both measures declined rapidly over the ensuing 5 d to the lowest measured in the study, and the bivalves began to die. Hemolymph oxygen content fell dramatically at 9°C to values between 2% and 12% of ambient water O2 content and had a maximum Po2 of around 20 mmHg. These data indicate an experimental upper lethal temperature of 9°C and a critical temperature, where a long-term switch to anaerobic metabolism probably occurs, of around 6°C for L. elliptica. Concurrent measures of mitochondrial function in the same species had indicated strong thermal sensitivity in proton leakage costs, and our data support the hypothesis that as temperature rises, mitochondrial maintenance costs rapidly outstrip oxygen supply mechanisms in cold stenothermal marine species.