For the first time, in vivo heat-exposure experiments were conducted on the hydrothermal vent polychaete Hesiolyra bergi from the hottest part of the vent biotope. Using a pressurised incubator equipped with video-facilities, we found that H. bergi, which forages around and in the tubes of the thermophilic Alvinella sp., became hyperactive once temperature exceeded 35 degreesC and further lost co-ordination in the 41 to 46 degreesC interval, just before death occurred. Another exposure experiment at 39 degreesC for 3 to 4 h led to 80 % mortality (max) 9 h after heat shock, and 100 % thereafter. In view of the much higher temperatures recorded in this organism's habitat, these results suggest that tolerance to high temperatures (exceeding 40 degreesC) is not a pre-requisite for life amongst alvinellid tubes. Behavioural responses (escape from heat) may suffice.