Formulation of cost-effective preventive control programmes for helminth infection in small ruminants should be based on sound epidemiological knowledge of the time relationship between contamination of pastures and the seasonal availability of infective larvae in a given geographic area. Epidemiological data generated from an on-station experiment involving the use of tracer lambs and monitoring of pasture contamination levels in the central Ethiopian highlands clearly showed a distinct seasonal pattern of nematode infection in sheep.