Temperature dependent changes in the mode of energy metabolism and in acid-base status were studied in the range from -1.7 to 26°C in two populations of Arenicola marina collected in summer as well as in winter from intertidal flats of the North Sea (boreal) and the White Sea (subpolar).Extreme temperatures led to an accumulation of anaerobic end products, indicating the existence of both a low and a high critical temperature (Tc), beyond which anaerobic metabolism becomes involved in energy production. In summer animals from the North Sea the high Tc was found at temperatures above 20°C, and the low Tc below 5°C. Latitudinal or seasonal cold adaptation lead to a more or less parallel shift of both Tc values to lower values.Between critical temperatures intracellular pH declined with rising temperature. Slopes varied between -0.012 and -0.022 pHi-units/°C. In summer animals from the North Sea, the slope was slightly less than in White Sea animals, but differences appeared independent of the season. However, slopes were no longer linear beyond critical temperatures. A drop in intracellular pH at low temperatures coincided with the accumulation of volatile fatty acids in the body wall tissue of North Sea animals. A failure of active pHi-adjustment is held responsible for the reduced ÆpHi/ÆT at temperatures above the high Tc. Extracellular pH was kept constant over the whole temperature range investigated.The ability of North Sea animals to adapt to temperatures beyond the Tc is poor as compared to White Sea specimens. The larger range of temperature fluctuations at the White Sea is seen as a reason for the higher adaptational capacity of the subpolar animals. A hypothesis is developed that, among other mechanisms, Tc values are set by an adjustment of mitochondrial density and thus, aerobic capacity.