The role of yeast supplementation in small ruminants on rumen fermentation, animal performance, and nematode control is still unclear. The effect of Celmanax ® (yeast culture product) in two diets using lambs on body weight gain, rectal temperatures and nematode loads were monitored. 24 sheep (17 ± 2.8 kg BW), housed in individual pens, were divided into four treatments and fed on four basal diets; lucerne meal (LC), lucerne meal plus Celmanax ® (LCC), sunflower meal (SF), and 4 sunflower meal plus Celmanax (SFC), for 96 days including 14 days adaptation. Dry matter intake, average body weight gain and feed conversion efficiency were calculated. Rectal temperatures and nematode egg count/gram (EPG) were measured once a week. Feed intake was higher (P < 0.05) in LCC and SFC than in LC and SF, respectively. Overall, weight gain was higher in SFC (P < 0.05) than SF. Feed conversion efficiency was higher (P < 0.05) in LCC than LC. Nematode EPG were lower (P < 0.05) in Celmanax ® supplemented diets (LCC and SFC) than in their controls. Rectal temperatures dropped (P < 0.05) in Celmanax ® diets. These findings suggest that, Celmanax ® has the potential of improving animal performance and may be used in conjunction with other anthelmintics to control nematodes.