Trichoderma harzianum is a filamentous fungus that has been proposed as a potential biocontrol agent against phytopathogenic fungi [1] and more recently as opportunistic, avirulent plant symbiont [2]. The antagonism by T. harzianum has been explained by different mechanisms [3]. One of them, mycoparasitism, involves the production of several hydrolytic enzymes for the local degradation of the host fungal cell wall and further penetration inside its hyphae as main steps [1].Several mycoparasitic strains included in different taxonomic groups in the Trichoderma genus [4,5] secrete complex sets of enzymes [6]. Within these enzymes we can find hydrolytic activities able to degrade most components of fungal cell walls (chitinases, glucanases, proteases, lipases, etc.). These are usually present as isozyme groups composed by proteins with the same activity but different catalytic and molecular properties [7][8][9][10][11][12].Chitinases and b-1,3-glucanases are considered the main enzymes responsible for the degradation of the host cell walls, as chitin and b-1,3-glucan are their two major components. However, other enzymes hydrolyzing less abundant, but structurally important components (as b-1,6-glucan), can contribute to the efficient disorganization and further degradation of the cell wall by Trichoderma. b-1,6-glucan has been described in budding yeasts as the link between cell wall proteins and the main b-1,3-glucan ⁄ chitin polysaccharide [13] supporting an important role for this polymer in the structure of the fungal cell wall.Although b-1,6-glucanases are widely distributed among filamentous fungi, few of them have been purified and characterized [10,[14][15][16][17] and few gene sequences have been published [18][19][20][21][22].We have previously described two b-1,6-glucanases in T. harzianum CECT 2413: BGN16.1 and BGN16.2 A new component of the b-1,6-glucanase (EC 3.2.1.75) multienzymatic complex secreted by Trichoderma harzianum has been identified and fully characterized. The protein, namely BGN16.3, is the third isozyme displaying endo-b-1,6-glucanase activity described up to now in T. harzianum CECT 2413. BGN16.3 is an acidic b-1,6-glucanase that is specifically induced by the presence of fungal cell walls in T. harzianum growth media. The protein was purified to electrophoretical homogenity using its affinity to b-1,6-glucan as first purification step, followed by chomatofocusing and gel filtration. BGN16.3 has a molecular mass of 46 kDa in SDS ⁄ PAGE and a pI of 4.5. The enzyme only showed activity against substrates with b-1,6-glycosidic linkages, and it has an endohydrolytic mode of action as shown by HPLC analysis of the products of pustulan hydrolysis. The expression profile analysis of BGN16.3 showed a carbon source control of the accumulation of the enzyme, which is fast and strongly induced by fungal cell walls, a condition often regarded as mycoparasitic simulation. The likely involvement b-1,6-glucanases in this process is discussed.