The shrimp Rimicaris exoculata swarms over high-temperature chimneys at Mid-Atlantic Ridge hydrothermal sites, thriving in a mixture of hot vent fluid (350 degrees C) and cold seawater (2 degrees C). It may, therefore, be subjected to a thermal regime that is assumed to be stressful for animals. Evaluating the stress status of species in their natural environment is a key issue for understanding the thermal biology of metazoans. In the case of deep-sea species, attempts to quantify natural levels of stress proteins are obviously impaired by deleterious decompression effects during sampling. Here we quantified the heat shock protein (hsp70) mRNA expression of shrimps sampled at a depth of 2300 m and recovered at their native pressure. These shrimps were not heat-stressed, as witnessed by their very low level of heat-inducible hsp70, while an important response was evidenced after an experimental 30 degrees C exposure. Our data also indicate that R. exoculata was nevertheless attracted by the temperature of fluid emission, and would deal with the harsh thermal conditions through microhabitat selection close to its thermal preferendurn (10 to 25 degrees C).