The dominant parasites in the Karoo are Nematodirus spathiger (Railliet, 1896) and Trichostrongylus falculatus Ransom , 1911 . In the moister eastern regions Haemonchus contortus (Rudolphi, 1803) and Oesophagostomum columbianum (Curtice, 1890) occur but they decrease markedly as the region becomes more arid. Ostertagia circumcincta (Stadelmann , 1894) is of little or no significance. The free-living stages of N. spathiger are highly resistant to heat and desiccation, but T. falculatus cannot survive if the mean monthly mean temperatures exceed 20°C and even if the monthly rainfall exceeds 50 mm, there is but a slight increase in worm burdens. Both species reach peak worm burdens in winter. The presence of H. contortus or possibly 0. columbianum has a deleterious effect on N. spathiger. Strategic drenching is recommended in March and July and tactical drenching when climatic conditions are favourable.