The development and survival of larvae of Nematodirus filicollis, an important nematode parasite of ruminants, were extensively investigated in the laboratory for the first time, using eggs harvested from sheep grazing in the UK in winter. Third-stage larvae (L3s) developed within the eggs between 12 and 25 degrees C, with the proportion of eggs developing successfully being highest at the upper end of this range. Eggs required chilling below 10 degrees C before being able to hatch at all, and maximum hatch was observed only following chilling to 4 degrees C. Hatching occurred above 6 degrees C, and peaked at 13 degrees C, with the proportion of eggs hatching decreasing to 17-20 degrees C, which appears to form an upper threshold temperature for hatching. When fully developed eggs were placed at 11-13 degrees C, L3 emerged very rapidly with hatching being completed within 6 days. Mortality of emerged L3s increased with temperature, particularly above 20 degrees C. For this parasite, given the low minimum hatching thresholds and the rapid emergence of L3, the existence of an upper hatching threshold is likely to have limited impact on patterns of larval availability on pasture. The strict requirement for chilling is surprising, being at odds with both previous observations and the wide geographical distribution of this species. It is possible that the proportion of eggs requiring chilling is an adaptable trait, enabling early infection of naïve hosts in the face of competition from N. battus, yet also persistence in regions that do not provide good conditions for the chilling of eggs. Further study of selectable variation in this trait could help us to understand the contrasting geographical ranges and disease patterns within the genus, and how different species might respond to climate change.