Adaptive flexibility under temperature stress was studied in a boreal and a subpolar population of Arenicola marina (L.) from the North and White Seas. Temperature stress leads to the onset of anaerobic metabolism and the ability to acclimate is interpreted to be associated with a reversal of this initial transition to anaerobiosis, restoring total aerobic conditions. Thus, end products of anaerobic metabolism as well as adenylate and ammonium concentrations were measured as indicators of temperature stress. The results reveal that the populations differ in their ability to acclimate to temperatures 4°C above their respective high critical temperature. Lugworms from the North Sea survived for only slightly longer than 1 d, while concentrations of alanopine and strombine were significantly elevated. However, adenylate and ammonium levels remained close to control values. In contrast, White Sea specimens showed drastic changes in the concentrations of anaerobic end products and ammonium, and ATP pools were depleted to levels below detection limits during the first 24 h at high temperatures. A higher metabolic proton production in White Sea worms, together with a lower intracellular non-bicarbonate buffer capacity led to a drastic drop in intracellular pH, whereas acidosis was only moderate in North Sea worms. Unexpectedly, the subpolar worms not only survived heat exposure, but even exhibited an adaptive reversal of anaerobic metabolic changes. Whereas heat exposure was compensated in White Sea worms only, the short term ability to survive cold stress (-1.7°C) is limited in summer worms from both populations. Therefore, after an initial reversal of anaerobic metabolism, concentrations of volatile fatty acids and succinate increased again after 3 d. Since White Sea worms can acclimatise to temperatures below -2°C during winter, our data suggest, that in contrast to warm acclimation, cold acclimation requires a moderate and protracted decline in temperature.