Alvinellid polychaetes are, to date, restricted to deep-sea hydrothermal vents of the eastern and the western ridges of the Pacific Ocean. These organisms Live in various sulfide-rich habitats, including the hottest part of the hydrothermal environment (i.e. chimneys). They experience transient anoxia, high levels of heavy metals and H2S, natural radioactivity and temperatures ranging from 5 to 80 degrees C which vary greatly with time. The Alvinellidae, as many vent organisms, have developed specific adaptations to cope with this harsh and unstable environment. Enzyme systems are good markers of the adaptation of ectotherms to temperature, which acts on both enzyme kinetics and protein denaturation. We estimated genetic distances between 11 alvinellid species using a data set of allozymes and studied in vitro allozyme thermostabilities of aspartate-amino transferase (AAT), glucose-6-phosphate isomerase (GPI) and phosphoglucomutase (PGM), which may play a role in orientating aerobic versus anaerobic metabolism pathways, for 8 species using the most common homozygous genotypes. Results show great genetic divergences between species living in distinct microhabitats as well as strong thermostability differences within and between species which also rely on different enzymatic strategies (phenotypic plasticity versus genetic variability). Allelic fitness to temperature in a highly fluctuating environment may explain the high level of polymorphism found in alvinellids and may have also provided sufficient genetic divergence between individuals living in distinct thermal regimes to produce speciation.