The influence of four levels of dietary supplementation on the development of the immune response and on the biology of Haemonchus contortus and Trichostrongylus colubriformis was evaluated in Dorper lambs under serial artificial mixed-infections with 1000 infective larvae (L3) of each species, given every three days for 12 weeks. For each of the four diets formulated, one infected group (n=7) and one control group (n=4) were set up. Diets 1, 2, 3 and 4 contained 0%, 25%, 50% and 75% of concentrate, respectively. There was an evident immune response to infection in all groups, manifested through eosinophilia and production of anti-L3 immunoglobulins. There was a significant inverse relationship between the levels of nutrients in the diets and the numbers of late fourth-stage larvae, early fifth-stage larvae and adults of H. contortus. Most of the H. contortus parasite population was in hypobiosis at the early-fourth stage, representing 59.5%, 75.8%, 83.3% and 86.3% of the total Haemonchus worm burden, respectively, in the Diets 1, 2, 3 and 4. In the case of T. colubriformis, hypobiosis was not observed and the diet had no influence on establishment of the parasite.