<jats:title>SUMMARY</jats:title> <jats:p>Mitochondrial respiration, energetic coupling to phosphorylation and the production of reactive oxygen species (ROS) were studied in mitochondria isolated from the eurythermal bivalve Mya arenaria (Myoidea) from a low-shore intertidal population of the German Wadden Sea. Measurements were conducted both within the range of the habitat temperatures (5-15 °C) and when subjected to heat exposure at 20 °C and 25 °C. Experimental warming resulted in an increase in the rate of state 3 and state 4 respiration in isolated mitochondria. The highest respiratory coupling ratios (RCR) were found at 15 °C; at higher temperatures mitochondrial coupling decreased,and release of ROS doubled between 15 and 25 °C. ROS production was 2-3%of total oxygen consumption in state 3 (0.3-0.5 nmol ROS mg-1protein min-1) at the habitat temperature, reaching a maximum of 4.3 % of state 3 respiration and 7 % of oligomycin-induced state 4+respiration under heat stress. Thus, state 4 respiration, previously interpreted exclusively as a measure of proton leakage, included a significant contribution from ROS formation in this animal, especially under conditions of heat stress. Oxygen radical formation was directly dependent on temperature-controlled respiration rates in states 3 and 4 and inversely related to mitochondrial coupling (RCR+) in state 4. Mitochondrial ROS formation is therefore involved in cellular heat stress in this eurythermal marine ectotherm.</jats:p>