Measurements of body temperatures in the field have shown that spatial and temporal patterns are often far more complex than previously anticipated, particularly in intertidal regions, where temperatures are driven by both marine and terrestrial climates. We examined the effects of body size, body position within the sediment, and microhabitat (presence or absence of Spartina alterniflora) on the body temperature of the mussel Geukensia demissa. We then used these data to develop a laboratory study exposing mussels to an artificial "stressful" day, mimicking field conditions as closely as possible. Results suggested that G. demissa mortality increases greatly at average daily peak temperatures of 45 degrees C and higher. When these temperatures were compared to field data collected in South Carolina in the summer of 2004, our data indicated that mussels likely experienced mortality due to high-temperature stress at this site during this period. Our results also showed that body position in the mud is the most important environmental modifier of body temperature. This experiment suggested that the presence of marsh grass leads to increases in body temperature by reducing convection, overwhelming the effects of shading. These data add to a growing body of evidence showing that small-scale thermal variability can surpass large-scale gradients.