SUMMARYMarine turtles are globally threatened. Crucial for the conservation of these large ectotherms is a detailed knowledge of their energy relationships, especially their at-sea metabolic rates, which will ultimately define population structure and size. Measuring metabolic rates in free-ranging aquatic animals, however, remains a challenge. Hence, it is not surprising that for most marine turtle species we know little about the energetic requirements of adults at sea. Recently, accelerometry has emerged as a promising tool for estimating activity-specific metabolic rates of animals in the field. Accelerometry allows quantification of the movement of animals (ODBA/PDBA, overall/partial dynamic body acceleration), which, after calibration, might serve as a proxy for metabolic rate. We measured oxygen consumption rates (V O2 ) of adult green turtles (Chelonia mydas; 142.1±26.9kg) at rest and when swimming within a 13m-long swim channel, using flow-through respirometry. We investigated the effect of water temperature (T w ) on turtle V O2 and tested the hypothesis that turtle body acceleration can be used as a proxy for V O2 . Mean massspecific V O2 (sV O2 ) of six turtles when resting at a T w of 25.8±1.0°C was 0.50±0.09ml min . sV O2 increased significantly with T w and activity level. Changes in sV O2 were paralleled by changes in respiratory frequency (f R ). Deploying bi-axial accelerometers in conjunction with respirometry, we found a significant positive relationship between sV O2 and PDBA that was modified by T w . The resulting predictive equation was highly significant (r 2 0.83, P<0.0001) and associated error estimates were small (mean algebraic error 3.3%), indicating that body acceleration is a good predictor of V O2 in green turtles. Our results suggest that accelerometry is a suitable method to investigate marine turtle energetics at sea.