In their natural habitat, animals are exposed to a variety of stress factors, including extreme temperatures, low water availability, and toxic stress from chemical pollutants. In this study we examined the interaction between realistic environmental levels of soil-copper contamination and realistic winter temperatures on survival of the cosmopolitan freeze-tolerant earthworm Dendrobaena octaedra. These interactions were tested using a full factorial design with six copper concentrations between 0 and 200 mg Cu/kg dry soil and five temperatures from +2 to -8 degrees C. A highly significant synergistic interaction existed that demonstrates that exposure to subzero temperatures significantly reduced copper tolerance and, conversely, that copper exposure significantly reduced freeze tolerance. Copper had no effect on glucose production, which is believed to be a major component of the cryoprotective system and the only known cryoprotectant in D. octaedra. This points to other mechanisms behind the observed synergy, possibly impaired osmoregulatory function of the cell membrane. The results support the working hypothesis that interactions between toxicants and dominant natural stress factors can alter the organisms' tolerance to these individual stressors.