An experiment consisting of two periods (P1 and P2) was conducted to determine the effect of adding a probiotic of bacterial strains of Bacillus subtilis and B. licheniformis to a basal diet of low quality grass hays on voluntary consumption (VC) and digestibility. Ten Creole lambs were randomly assigned to one of two treatments: control (without additive) or probiotic (with additive In diet). The basal diet consisted of a dally forage offering equal to 4% of live weight (LW) on a dry matter (DM) basis. The forage offered was 50% native grass hay [71.7% neutral detergent fiber (NDF), 4.9% crude protein (CP) in P1; and 71.2% NDF, 5.4% CP in P2], and 50% of Hyparrhenia rufa hay (78.8% NDF, 3.5% CP in P1; and 75.6% NDF, 5.5% CP in P2). The lambs were supplemented with 225 g of commercial concentrate (CC) daily. The additive was mixed with the CC to supply 1.33 X ICcfu/head daily during the 49 days of P1. From day 50 to 84 (P2), the probiotic addition was suspended to determine possible residual effects. The lambs were weighed weekly. The VC and digestibility of DM, NDF and CP were determined from day 42 to 49 in P1 and from day 77 to 84 in P2. The variables related to parasitism and anemia: fecal egg count (FEC), FAMACHA® index score and packed cell volume (PCV) were determined every 21 days. Data from each experimental period were analyzed according to a completely randomized design with five replicas per treatment. During P1, the daily LW gain of the lambs was 23 vs. 20 g for control and probiotic treatments, respectively. The dietary addition of probiotic increased (p < 0.05) total DM intake (445 vs. 484 g/d), DM intake as a percentage of LW (2.04 vs. 2.37) and forage DM as a percentage of total DM intake (54.77 vs. 59.42). The digestibility coefficients of DM (59.98 vs. 62.62%) and CP (59.35 vs. 61.76%) did not differ between treatments, but there was a tendency (p = 0.09) to improve NDF digestibility (58.71 vs. 62.48%) with probiotic addition. The FEC observed in the control and probiotic groups were 820 vs. 1,380 eggs/g initially and increased more in the control (p < 0.05) to 2,390 vs. 2,780 eggs/g at day 21, then decreased less in this group to 1,830 vs. 1,480 eggs/g at day 42. The PCV values changed between days 0 and 42 from 24.4 to 17.9% in the control and from 20.6 to 22.6% in the probiotic group, but without significant effects (p > 0.05). The anemia level according to FAMACHA® score differed little between treatments and did not exceed a maximum of 2.6. During P2, the LW gain of the lambs of both control and previously probiotic treated groups was 48 g/d. Total DM intake was 587 vs. 562 g/d and digestibilities were: DM (58.46 vs. 57.59%), NDF (57.50 vs. 56.85%) and CP (60.78 vs. 62.11%) without significant differences (p > 0.05). The FEC decreased progressively to respective final values at 84 days of 1,230 vs. 440 eggs/g, whereas the PCV increased to 23.4 vs. 25.1% at day 84 without significant differences. The maximum FAMACHA® score was 2.8 vs. 2.2 in the two respective treatments. In summary, the addition of the probiotic in the diet improved VC and tended to increase NDF digestibility, but did not affect growth, even though there were signs of animal health benefiting. After suspending the use of the additive, no residual effect on the variables evaluated was observed.