The catalytic core domains of cellobiohydrolase I (CBHI) and endoglucanase I (EGI) from Trichoderma reesei have been crystallized using the hanging drop vapour diffusion method. In the case of CBHI, use of polyethylene glycol 20,000, and calcium chloride at low pH produced good quality single crystals suitable for X-ray studies. The crystals belong to a primitive orthorhombic space group with unit cell dimensions a = 84·0 Å, b = 86·2 Å, c = 111·8 Å, and diffract beyond 2·0 Å resolution. Bipyramidal crystals of EGI core were grown from ammonium sulphate at pH 7·5. The crystals are tetragonal, either P 4122 or the enantiomorph P4322, with cell dimensions a = b = 101·8 Å and c = 198·0 Å, and at best diffract to a resolution of 2·5 Å.