Abstract: The effect of a commercial beta-Mannanase enzyme (CTCZYME; CTC Bio Inc., Seoul, Korea) on broiler performance and dry matter output in corn-soybean meal diets was investigated. One hundred and twenty one-day-old male chicks of a commercial broiler strain were fed for 19 d on a nutritionally adequate diet based on corn and soybean meal. At that time birds were randomly allocated to four treatments, each of which had six replicates of five birds each. Aliquots of the basal diet were supplemented with four levels of the CTCZYME: 0%, 0.025%, 0.05% (recommended level) and 0.1%. Chromic oxide was used as an indigestible marker. After eight days of acclimation excreta was collected daily, weighed and dried at 130°C for 24 h to obtain the dry matter output. After seven days of excreta collection the birds were weighed and the experiment was terminated. There were no significant differences for body weight gain, feed conversion or feed intake. The addition of CTCZYME at the inclusion level of 0.05% and 0.1% significantly reduced (p<0.002) the daily dry matter excreta output per bird. Analysis of the excreta showed a reduction of the nitrogen level as the level of CTCZYME increased, indicating an improvement in nitrogen utilization by the bird. Gross energy of the excreta decreased as the inclusion level of the enzyme increased. When the inclusion levels of the enzyme increased, calcium and phosphorus levels increased, possibly due to a concentration effect. Higher levels of chromium in the excreta were observed with the increment of the enzyme, suggesting an improvement on the digestibility of the feed product of the effect of the beta-mannanase. These data indicate that nutrient digestibility is enhanced by the effect of CTCZYME. The reason for the increasing digestibility not affecting the broiler performance may be due to the short time the enzyme was included in the diet. Another possibility is that a change in carcass composition may have occurred; more protein could have been deposited on the carcass instead of fat when the enzyme was included but further studies are required to confirm this assumption. Also, it has to be taken into account that the dietary protein levels provided the needs for the bird and they probably did not need to assimilate the now more available amino acids due to the effect of CTZyme.