The filamentous fungus Hypocrea jecorina secretes profuse quantities of cellulases and hemicellulases. These enzymes act in an orchestrated fashion on plant biomass and degrade this into monomeric or oligomeric sugars. Among all those glycoside hydrolases produced by the fungus a β-xylosidase (Bxl1) can be found that belongs to glycoside hydrolase family (GH) 3, Bxl1 is an glycoside hydrolase that mainly hydrolases unbranched xylans, glucuronoxylans, and β-1,4-xylooligosaccharides, into the reaction product D-xylos. Xylan is after cellulose the second most abundant polysaccharide in nature, accounting for approximately one third of all renewable organic carbon. Because of its abundance, there has been an increasing industrial interest in utilizing xylans and the enzymes hydrolyzing these in a wide range of industrial applications such as; supplement in animal feed, bleaching of cellulose pulp, in the production of food drinks, ethanol and xylitol. In this paper, we will present the structure of the first GH3 β-xylosidase, Bxl1 from H. jecorina. The structure of Bxl1 has been determined to 1.8Å resolution by X-ray crystallography. The overall structure of Bxl1 to a large extent resembles the structures of the only two members of GH3 with known three-dimensional structure (barley β-D-glucan exohydrolase and Vibrio cholera β-hexosaminidase), i.e a N-terminal (α/β)₈ TIM barrel domain followed by a (α/β)₆ sandwich domain. Results from small-angle X-ray scattering (SAXS) and circular dichroism spectroscopy (CD) experiments have shown that Bxyl1 has an additional third domain. The crystal structure of Bxyl1 clearly shows the presence of an extra domain located at the C-terminus.