Since their development, farmers have become heavily reliant on anthelmintics for the effective control of nematode parasites in their lambs and other grazing livestock. However, alternative approaches in the control of nematode parasites now need to be found due to the increasingly widespread development of anthelmintic resistance (Athanasiadou et al., (2001), Butter et al., (2000)). This paper reports an in vivo study carried out to determine the effectiveness of quebracho tannins as an alternative to anthelmintics in the control of gastrointestinal nematode parasites in lambs.