Abstract
The free-living phase of the life-cycle of Nematodirus helvetianus was studied out of doors on grass plots and the effects of temperature and humidity on the free-living stages were studied in the laboratory. The results of these observations are discussed in relation to the bionomics of N. battus and N. filicollis, and tentative deductions made regarding the epidemiology of N. helvetianus infection.I wish to thank Mr J. Bailey for technical assistance.