Intestinal mucosal morphology, topography and populations of crypt intraepithelial globule leucocytes (IGLs), goblet cells and mucosal mast cells were compared in 4 groups of 3-month-old lambs which had been reared worm-free from birth. Groups 1 and 2 were infected daily on 5 days per week with 2500 Trichostrongylus colubriformis larvae for 18 weeks. Group 2 lambs were treated with fenbendazole (5 mg per kg) at week 10 and week 15. Group 3 lambs were infected as above with T. colubriformis for 10 weeks, dosed with fenbendazole and given no further lavae. A fourth group of 4 lambs (Group 4) was maintained worm-free throughout the experiment as controls. All lambs were killed at week 20. Worm populations (range 410 to 30460) and mucosal damage varied considerably between individuals in the untreated Group 1 lambs, but damage was severe, with total villous atrophy, in lambs with large worm burdens. In 3 lambs with moderate worm burdens, worms were seen only in circumscribed flat “finger-print” lesions in the jejunal mucosa. There was no correlation between IGL numbers, mast cell numbers and final worm burdens in Group 1 lambs, though there was a trend towards an association between high worm populations and low IGL numbers. Only trivial pathological changes were detected in the re-infected Group 2 lambs. Low worm egg counts were found in this group 4 weeks after the first anthelmintic treatment, and virtually no eggs were present after the second treatment. Mucosal morphology in Group 3 lambs, which were not re-infected, resembled the controls at the end of the experiment. IGL numbers were greater in both treated groups than in controls, and numbers in the re-infected Group 2 lambs were greater than those in Group 3 lambs. Mucosal mast cell numbers did not differ significantly between groups, but crypt goblet cell numbers were increased in all 3 infected groups, compared with controls.