The aim of this study was to evaluate the effect of the plant lectin concanavalin A (Con A) on gastrointestinal nematodes burdens in tropical lambs (West African breed - WA). The measures were taken weekly, feces samples for egg counts (eggs per gram of feces-EPG) and body weight (BW) of the animals. A completely randomized statistical model was used with 18 lambs with an average initial weight of 19.07 +/- 0.24 kg, divided into 2 groups (n = 9), balanced according to initial weight and parasitic load. The treatments were T0 (animals without Con A or control) and T1 (animals dosed with Con A at a rate of 6 mg/kg live BW). Animals received supplementation every morning before grazing, with 1.5% BW of a feed containing 60% of brewery yeast and 40% of concentrate (18% CP). T1 animals received oral doses of Con A, using a syringe, before supplementation, while the control animals received oral dosing of tap water, at the same time. Once consumed the supplement, all animals grazed until 16: 00 hours (h). To evaluate EPG, stool samples were taken on a set day and time, once a week and analyzed by the McMaster technique. Analysis of variance with repeated measurements over time were performed using SAS statistical software (PROC MIXED). EPG were logarithmic transformed using base 10, after they were backtransformed to obtain average values. The results indicate a dose effect of lectin on EPG (P < 0.0001) with 588.8 and 145.3 for T0 and T1, respectively. The interaction of treatment per week resulted highly significant (P < 0.0001). T1 produced a steady decline on EPG. Weights were 23.02 +/- 2.5 kg vs. 22.19 +/- 1.5 kg for T1 and T0, respectively (P < 0.0001), yielding a difference of 840 g between both treatments in favour for treated animals. It can be concluded that there was a positive effect of using lectin Con A, both in the control of parasitic loads and productive performance, as the T1 was favoured in both cases.