Infestation of gastrointestinal nematodes is a major problem in grazing animals. Control is achieved through administration of anthelmintics; however, because of indiscriminate use, there have been increased reports of resistance to chemical anthelmintics which led to the failure of parasite control. This study determined the efficacy of the chlamydospore of Duddingtonia flagrans as biological control against common strongyle roundworms of buffaloes. Using corn meal agar assay, strongyle infective larvae were treated with and without D. flagrans. The chlamydospore/ gram (CG) assay tested a dose-dependent concentration wherein feces with 2,100 eggs/ gram (EPG) strongyles were treated with D. flagrans at an increasing doses of CG (100,000, 250,000 and 500,000). Results showed an 84.39% larval reduction after treatment with 500,000 CG. The chlamydspore/ egg assay evaluated increasing ratios of egg to chlamydospore dose (1:0, 1:100, 1:500, 1:1000) using the 2,100 EPG feces. The ratio 1:500 achieved the highest percent larval reduction (78.88%). D. flagrans was directly fed to buffaloes at varying concentrations (50,000, 150,000, 250,000 chlamydospores/kg BW). A 78.77% larval reduction was observed at 50,000 chlamydospore/kg BW oral administration for 5 days. This study showed the efficacy of D. flagrans as a potential alternative for anthelmintics in buffaloes.