Faeces containing eggs of Hyostrongylus rubidus were deposited on grass plots each month throughout 1980. Eggs in faeces deposited on plots from May to October developed into infective larvae but in May development was completed only in artificially moistened faeces, the weather was warm and dry and natural faeces dried out rapidly resulting in the death of eggs and pre-infective larvae. For the remainder of this period the weather was sufficiently warm and wet for development to be completed. More eggs developed into infective larvae on plots with well-grown herbage than on plots with short herbage. In April a few eggs survived and developed into lst-stage larvae but no further development took place. In the laboratory development was completed at temperatures ranging from 10 to 27 °C but no development took place at 4 °C. Out-of-doors infective larvae survived on herbage for up to 10 months while in the laboratory, infective larvae suspended in tap water survived even longer at 10 and 22 °C, but the larvae were rapidly killed by continuous freezing and by desiccation when the relative humidity was less than 95%. The relationship between climatic conditions and the development and survival of the free-living stages is discussed and a comparison made with the free-living stages of Oesophagostomum dentatum.