The seasonal incidence of nematode infestations in pigs raised under semi-intensive conditions was determined by the monthly slaughter of 2--4 tracer pigs exposed to infestation in an earthernfloored pen for periods of 1 or 2 months. Although worm burdens were generally small, Ascarops strongylina appeared to be more prevalent from November to March than during the other months of the year. Once Ascaris suum became established in the pen nearly all the pigs became infested. The number of worms never exceeded 88, however, and no seasonal incidence pattern could be determined. Trichuris suis favoured the warmer months from November to March, the smallest numbers being recovered during September and October. One pig only harboured Trichostrongylus colubriformis and 3 had Oesophagostomum dentatum.