The seasonal incidence of helminth infestation on a farm in the Transvaal Highveld was determined by the slaughter of tracer lambs exposed on pasture for periods of approximately 6 weeks. Haemonchus contortus, the most prevalent nematode, was recovered in the greatest numbers from January- May 1974. Peak counts of Ostertagia circumcincta were made during March and April 1974 and in January 1975, while Trichostrongylus spp. were mainly encountered from March-June. Cooperia spp. were generally recovered from November-May and Moniezia expansa from October-April. H. contortus, O. circumcincta and Cooperia spp. all exhibited increasing degrees of arrested development in the 4th larval stage from February-June.