The influence of seasonal and acute temperature changes on mitochondrial functions were studied in isolated mitochondria of the eurythermal lugworm Arenicola marina (Polychaeta), with special emphasis on the interdependence of membrane potential and radical production. Acclimatisation to pre-spawning/ summer conditions is associated with rising mitochondrial substrate oxidation rates, higher proton leakage rates, elevated membrane potentials, and increased production of reactive oxygen species (ROS) in isolated mitochondria compared with mitochondria from winter animals. However, a high ROS production was compensated for by higher activities of the antioxidant enzymes catalase and superoxide dismutase, as well as lower mitochondrial densities in summer compared with winter animals. In summer animals, a higher sensitivity of the proton leak rate to changes of membrane potential will confer a better flexibility for metabolic regulation (mild uncoupling) in response to temperature change. These seasonal alterations of mitochondrial functioning support modifications of energy metabolism in eurythermal and euryoxic organisms on intertidal mudflats during summer. In winter, low and less changeable temperatures in intertidal sedimentary environments, allow for higher respiratory efficiency at low aerobic metabolic rates and lower membrane potentials in A. marina mitochondria.