South America has a large population of small ruminants. Currents estimates are approximately 100 million sheep and 23 million goats. A large percentage of these flocks are raised in the humid tropics/sub-tropics. Nematode parasitism is singly the most important disease of these animals and typically farmers resort to frequent anthelmintic treatment in attempts to control this problem. Because of this reliance on drugs, price consideration is an important determinant in a farmer's choice of anthelmintic. In some circumstances, this has led to unethical practices of drug adulteration and substitution, or the offering to farmers of cheap, sub-standard generic products. The prevalence of anthelmintic resistance has not been investigated in any widespread sense in South America, although some of the first reports of resistance were made in southern Brazil almost 30 years ago. The following series of papers outline surveys conducted in the humid topics/sub-tropics of southern Latin America to assess the significance of resistance to the broad-spectrum anthelmintic groups in nematode parasites of sheep flocks.