ABSTRACT: Animals living at deep sea hydrothermal vents experience high values of both temperature and hypoxia/sulfide. In these harsh environments polychaetes represent an important proportion of the biomass and diversity. To determine the mechanisms facilitating survival, we investigated the activities and thermal stability of 6 enzymes functioning under anaerobic and aerobic conditions, as well as glycogen levels, in 2 paralvinellid species from hydrothermal vents. In addition, the enzyme activities of several polychaetes from hydrocarbon cold seeps, which encounter low oxygen and high sulfide, but not elevated temperature, were investigated for comparative purposes. ‘Anaerobic’ (lactate dehydrogenase, opine dehydrogenases) and ‘aerobic’ enzyme (citrate synthase) activities of hydrothermal vent paralvinellids were similar to those of shallow water polychaetes, and significantly higher than those in cold seep species. Surprisingly, we detected different metabolic pathways in various body parts of hot vent species. The branchiae were identified to be the body part in which aerobic metabolism likely dominates, whereas the body wall is the major focal point of anaerobic glycolysis. Enzyme activity levels remained nearly constant in paralvinellids maintained for up to 120 h in high-pressure chambers. Thermal stability observations of in vitro enzyme activities showed stability after exposure to elevated temperatures: up to 60°C for lactate dehydrogenase and up to 50°C for opine dehydrogenases, the latter correlating with the preferred temperatures of living paralvinellids. Heat shock experiments with live specimens revealed lethal temperatures of 45°C for Paralvinella palmiformis and 63°C for P. sulfincola. Heat shock treatments also reduced enzyme activity. Glycogen levels in vent species were lower than in shallow water species, suggesting that paralvinellids can obtain carbon through feeding under anoxic conditions.