The burden of infestation of the horn fly, Haematobia irritans (Linnaeus) (Diptera: Muscidae), differs among bovines within the same herd. We hypothesized that these differences might be related to the epidermal thickness of the cattle and the blood intake capacity of the fly. Results showed that dark animals carried more flies and had a thinner epidermis than light-coloured animals, which was consistent with the greater haemoglobin content found in flies caught on darker cattle. Similarly, epidermal thickness increased with body weight, whereas haemoglobin content decreased. Overall, we suggest that accessibility of blood is a factor that partially explains cattle attractiveness to flies.