The thermal limit for metazoan life, expected to be around 50 degrees C, has been debated since the discovery of the Pompeii worm Alvinella pompejana, which colonizes black smoker chimney walls at deep-sea vents. While indirect evidence predicts body temperatures lower than 50 degrees C, repeated in situ temperature measurements depict an animal thriving at temperatures of 60 degrees C and more. This controversy was to remain as long as this species escaped in vivo investigations, due to irremediable mortalities upon non-isobaric sampling. Here we report from the first heat-exposure experiments with live A. pompejana, following isobaric sampling and subsequent transfer in a laboratory pressurized aquarium. A prolonged (2 hours) exposure in the 50-55 degrees C range was lethal, inducing severe tissue damages, cell mortalities and triggering a heat stress response, therefore showing that Alvinella's upper thermal limit clearly is below 55 degrees C. A comparison with hsp70 stress gene expressions of individuals analysed directly after sampling in situ confirms that Alvinella pompejana does not experience long-term exposures to temperature above 50 degrees C in its natural environment. The thermal optimum is nevertheless beyond 42 degrees C, which confirms that the Pompeii worm ranks among the most thermotolerant metazoans.