The physiological adaptations required to survive exposure to low temperatures must be of great importance in allowing nematodes to colonize extreme terrestrial environments, such as Antarctic fellfields. Seasonal variation in the cold hardiness of three species of free-living nematodes was studied by monitoring supercooling points from approximately monthly samples over a period of 2 years. Two species, Teratocephalus tilbrooki Maslen and Plectus antarcticus de Man coexist within the relatively sheltered cushions and carpets of a moss, whilst the third species, Ditylenchus sp.B, is found in the more exposed aerial thalli of a lichen. Ditylenchus sp.B shows consistent supercooling ability, low enough (c -28⚬C) to avoid any significant incidence of freezing during the study period. Both of the moss-dwelling species, which exhibit bimodal supercooling point distributions with a high group at c -7⚬C and a low group at c -22⚬C encounter field temperatures likely to result in freezing. Only T. tilbrooki exhibits any degree of freeze tolerance. P. antarcticus is unique amongst these three species in showing a marked pattern of seasonal change in cold hardiness. Juvenile nematodes show greater supercooling ability than adults. Increased body pigmentation, possibly related to recent feeding activity, increases the likelihood of P. antarcticus freezing at high temperatures (> -10⚬C), but has no effect on the supercooling ability of Ditylenchus sp.B. Ditylenchus sp.B appears to be capable of preventing the accumulation of any nucleating agents which would cause freezing at high temperatures.