Although Nematodirus battus (Nematoda: Trichostrongyloidea) is an economically important and highly pathogenic parasite of sheep in the temperate regions, very little is known about the population dynamics of its free-living stages and their relationship with ambient temperature. Here we describe the temperature-related vital rates and thresholds of egg development, hatching and larval survival for the first time. N. battus eggs were able to develop between 11.5 and 27 degrees C, but development at the lower end of this range was more successful. Embryonated eggs did not hatch below 11 degrees C or above 17 degrees C. This is the first description of an upper threshold for hatching in trichostrongyloids. In contrast with most previous studies, although some eggs hatched only after being chilled, substantial proportions of eggs were also found to hatch without the need for chilling. These proportions were lower with increasing, within-hatching range, temperatures. Larval death rates significantly increased at temperatures towards and above the upper hatching threshold. The peculiar hatching behaviour of N. battus may therefore be explained in terms of optimization of larval survival. We argue that our findings confirm the likelihood of an arctic origin of the parasite. Probable changes and adaptations of parasite behaviour in the temperate regions, and the driving forces behind them, are discussed, as well as factors affecting persistence and geographical spread against a background of climate change.