Natural infestations of the cattle tick Boophilus microplus, levels of the buffalo fly Haematobia irritants exigua and faecal nematode egg concentrations (Bunostomum phlebotomum, Cooperia spp., Haemonchus placei, Oesophagostomum radiatum and Trichostrongylus axei) were assessed in 221 Belmont Red calves during the post-weaning period, when the animals were between 9 and 18 months of age. In addition, the 98 males of this group were challenged with B. microplus larvae on two separate occasions. There were strong positive correlations among replicate assessments of the same parasite. Field tick counts and tick counts following deliberate challenge were strongly correlated, and both showed negative correlations with post-weaning weight gain. There was a weak positive correlation between buffalo fly counts and post-weaning weight gain. There was a negative correlation between total worm egg count and weight gain. Among worm species, only the effect of O. radiatum on weight gain was significant. Cattle with bovine major histocompatibility (BoLA) antigens W6.1 and W7 had significantly fewer ticks than cattle lacking these antigens. Cattle with BoLA antigens W7 and CA36 had lower concentrations of nematode eggs in their faeces than cattle lacking these BoLA antigens.