Breeding sheep for resistance to gastrointestinal (GI) parasites has become an option to assist farmers to manage the growing anthelmintic-resistance problem in Australia. Field investigations have resulted in both favourable and unfavourable relationships between production traits and improved resistance to GI parasites. Two experiments, one with 10-month-old wethers, and the other with 18-month-old rams, were conducted in well controlled animal house environments to compare the Rylington Merino parasite resistant and unselected control lines. The wethers were fed at maintenance or 1.5 x maintenance, and the rams were fed ad libitum. The resistant sheep, compared with their unselected counterparts, had similar voluntary feed intake, digestibility and utilization efficiency of dietary nitrogen (N) and therefore, similar body weight change, body compositions and fleece weight. During the parasite challenge period there was an interaction between genotype and infection. The resistant sheep tended to grow more wool without parasite infection, whereas with the infection they responded, in terms of changes in feed intake, weight change and wool growth, faster and greater to the parasite challenge but they recovered more rapidly, so the overall performance during the infection remained similar to the control sheep. Resistance to parasites did not affect body weight gain of the sheep in these two experiments. The effect on wool production seemed to be age-related. Young sheep were more likely to suffer a reduction in performance and need more attention to nutrition, particularly in protein supply. Parasite infection reduced N utilization efficiency for wool growth. Increasing feed supply improved resilience of the sheep. (c) 2005 Elsevier B.V. All rights reserved.