Mitochondrial respiration, energetic coupling to phosphorylation and production of reactive oxygen species (ROS) were studied in mitochondrial isolates of the eurythermal bivalve Mya arenaria (Myoidea) from a low shore intertidal population of the German Wadden Sea. Measurements were conducted within the range of the habitat temperatures (5 to 15°C), as well as under heat exposure at 20 and 25°C. Experimental warming resulted in an increase of state 3 and state 4 respiration in mitochondrial isolates. Highest respiratory coupling ratios (RCR) were found at 15°C, while at higher temperatures mitochondrial coupling decreased significantly and ROS release doubled between 15 and 25°C. ROS production amounted to between 2 and 3% of total oxygen consumption in state 3 (0.3-0.5 nmol ROS /mg protein min) at habitat temperature, and to 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 protron leakage, was found to involve significant contribution of ROS formation in this animal, especially under heat stress conditions. Oxygen radical formation was directly dependent on temperature controlled respiration rates in states 3 and 4, and moreover inversely related to mitochondrial coupling (RCR+) in state 4. Mitochondrial ROS formation is obviously involved in cellular heat stress in this eurythermal marine ectotherm.