Abstract: Anthelmintic resistance against commonly used anthelmintics (ivermectin, levamisole, and fenbendazole) was studied in naturally occurring gastrointestinal (GI) nematodes in adult sheep flocks of Cuddalore district, Tamil Nadu by using Faecal egg count reduction tests (FECRT) and also to determine the efficiency of anthelminitics (fenbendazole, levamisole and ivermectin) used for treatment against nematode parasites. The results of the present study revealed high levels of anthelminitic resistance to Fenbendazole treated sheep flocks in Cuddalore district with the reduction of 50 - 89 per cent, whereas, a low resistant to Levamisole with the faecal egg reduction of 93 - 95 per cent. Ivermectin was found to be effective in controlling nematodes in all the sheep farms. The post-treatment (fenbendazole and levamisole) larval culture revealed the presence of Haemonchus contortus larvae.