The epidemiology of gastrointestinal helminth infections in beef cows and calves on pasture was studied in Belgium during the 1990 and 1992 grazing seasons. Weight gain, faecal egg counts, generic differentiation of infective larvae, serum pepsinogen levels, herbage larval counts and worm burdens of tracer calves were used as parameters. In Study 1 two groups of ten cows with their spring-born calves grazing on separate pastures (A and B) were monitored during the 1990 grazing season. Ostertagia ostertagi was the predominant species shed by cows and calves. Cows on Pasture A had significantly higher egg counts at turn-out than the B cows, creating a high pasture contamination in the autumn, evidenced by high Ostertagia worm burdens in the Pasture A tracer calves. Calves of both groups showed low egg counts (mean < 60 eggs g-1 faeces, EPG) throughout the grazing season. In Study 2 nine cow-calf pairs were monitored during the 1992 grazing season. The calves were born in winter or spring. Faecal egg counts of the cows remained low throughout the trial period. During the grazing season high egg counts were observed in the calves (mean up to 778 EPG). Cooperia oncophora was the predominant species in the calves. In the cows O. ostertagi, Oesophagostomum, C. oncophora and Trichostrongylus axei were present. It is suggested that, in the first study, the cows were the major source of pasture contamination, while in the second study the winter-born calves, being older and having a higher herbage intake resulting in a higher infection level, were largely responsible for the high Cooperia pasture infection level at housing.