The annelid Paralvinella grasslei is a deep-sea vent endemic species that colonizes the wall of active chimneys. We report here the first data on its thermal biology based on in vivo experiments in pressurized aquaria. Our results demonstrate that P. grasslei survives a 30 min exposure at 30 degrees C, and suggest that the upper thermal limit of this species is slightly above this temperature. The first signs of stress were noticed at 30 degrees C, such as a significant increase in the animal's activity and the expression of HSP70 stress proteins. A preliminary investigation of the kinetics of stress protein expression surprisingly showed high levels of HSP70 proteins as late as 3.5 h after the heat shock. Finally, we provide here the first sequences for vent annelid hsp70 (P. grasslei, Hesiolyra bergi and Alvinella pompejana). These constitute valuable tools for future studies on the thermal biology of these annelids.