Cellulose‐binding domains (CBDs) form distinct functional units of most cellulolytic enzymes. We have compared the cellulose‐binding affinities of the CBDs of cellobiohydrolase I (CBHI) and endoglucanase I (EGI) from the fungus Trichoderma reesei. The CBD of EGI had significantly higher affinity than that of CBHI. Four variants of the CBHI CBD were made in order to identify the residues responsible for the increased affinity in EGI. Most of the difference could be ascribed to a replacement of a tyrosine by a tryptophan on the flat cellulose‐binding face.