The shrimp Rimicaris exoculata swarms around hydrothermal black smoker chimneys at most vent sites along the Mid-Atlantic Ridge. This species maintains close proximity to the hydrothermal fluid, where temperatures can reach 350degreesC and steep thermal and chemical gradients are expected. We performed in vivo experiments in pressurized aquaria to determine the upper thermal limit [critical thermal maximum (CTmax)] of R. exoculata and to investigate some characteristics of the shrimp stress response to heat exposure. These experiments showed that the shrimp does not tolerate sustained exposure to temperatures in the 33-37degreesC range (CTmax). A heat-inducible stress protein belonging to the hsp70 family was identified in R. exoculata, and its synthesis threshold induction temperature is below 25degreesC. The R. exoculata optimal thermal habitat may thus be restricted to values lower than previously expected (<25&DEG;C).