Thermal stresses, such as those associated with global warming, have the potential to change the strength and even the sign of interspecific interactions. On rocky shores, the mussel Mytilus californianus is parasitized by shell-boring cyanobacteria. Cyanobacterial infestation weakens the shell, and can lead to mechanical failure and death. However, cyanobacterial infestation also causes the black outer layer of the shell to slough away. The resultant light gray shell absorbs less solar energy, and infested mussels remain cooler on sunny days. Manipulation of shell color in the field results in higher mortality in black-painted mussels relative to gray-painted mussels. Furthermore, following exceedingly hot weather, mortality is significantly lower for heavily infested, light-colored mussels than for lightly infested, dark colored mussels. Thus, shell-boring cyanobacteria act as mutualists during periods of intense thermal stress. This context-dependent symbiosis may allow mussels to occupy higher shore levels than would otherwise be possible, and thus indirectly benefit the hundreds of species which use mussel beds as habitat. Understanding the importance of thermal context in species interactions will be key in successfully predicting the outcomes of global climate change.