We examined the changes to populations of Trichostrongylus colubriformis and Teledorsagia circumcincta in mature sheep selected for reduced faecal worm egg count (WEC). Worm resistant (n = 19) and control (n = 10) genotype sheep were penned and dosed with a total of 10,000 T. colubriformis and 10,000 T. circumcincta per week for 18-weeks. Sheep genotypes were from lines previously bred over 15 years for either low WEC (resistant) or non-selected (control). Weekly WEC and the proportion of larvae from each species in faecal culture were measured during weeks 3-17. Egg hatchability was assessed on a pooled faecal sample from worm resistant or control genotype sheep at weeks 7, 9, 10, 13,14 and 17. At week 18 the number of adult and immature worms (early and late 4th, and 5th stage), and indicators of worm fecundity (female worm length and number of eggs in utero) were assessed at necropsy. Results show that resistant sheep had reduced mean WEC to only 18% of the control (P < 0.05) and increased the proportion of T. circumcincta larvae in faecal culture during weeks 8-17 (P<0.10). Egg hatch assays indicated a slight reduction in the viability of eggs from worm resistant genotypes at weeks 14 (P< 0.05) and 17 (P<0.10). At necropsy, resistant animals had 93% fewer adult T. colubriformis, 44% fewer adult T. circumcincta and had reduced indicators of fecundity in T. circumcincta by up to 40% (P< 0.05). We observed no change in the number T. circumcincta worms but an 11% increase in the proportion of early 4th stage T. circumcincta larvae in resistant animals (P<0.05). There were different temporal patterns in WEC and different prediction equations for WEC from necropsy traits for the two sheep genotypes (P< 0.05). Thus, our results suggest a changed host-parasite relationship in sheep selected for low WEC. We conclude that lower WEC is achieved through reduced number of adult worms for both species and reduced fecundity for T. circumcincta. These results support the hypotheses that worm resistant sheep with a strong immune function can regulate T. colubriformis by rejecting adult worms but that T. circumcincta is regulated through a combination of suppressed development and reduced female fecundity. (C) 2010 Elsevier B.V. All rights reserved.