The worm burdens of Angora and Merino wethers grazing with their own species or in a mixed flock were compared over a 4 month period. Based on faecal egg counts and larval differentiation, all animals had similar levels and types of infection at the beginning of the experiment when they were 15 months old. Although the initial infection in sheep and goats was similar, sheep subsequently developed a stronger resistance to worms. Therefore, at the termination of the experiment the sheep had significantly fewer worms of all species, except Nematodirus, than did the goats. There was no significant within-host difference in worm burdens whether the animals grazed exclusively with their own species or in the mixed flock.