1. The epizootiology of nematode parasites of sheep was investigated at the Outeniqua Experimental Farm, George, Cape Province. 2. There was a negative correlation between temperature and the availability of infective larvae on the pasture. During spring and summer when the mean average mean temperatures exceeded 20° C, low recoveries were recorded; whereas maximum recoveries were made when the mean temperature varied between 14° and 17° C in autumn and winter. 3. The major parasites at Outeniqua were Trichostrongylus spp., Ostertagia spp. and to a lesser degree H. contortus, Cooperia spp. and Nematodirus spp. 4. Flocks acquired massive worm burdens during the cooler months of the year, from autumn to spring. 5. The massive build-up of retarded fourth stage larvae and immature adults is a physiological process in the life-cycle, whereby the parasite ensures itself of optimal circumstances for survival against adverse climatic conditions.