Expression of Trichoderma reesei exo-cellobiohydrolase I (CBHI) gene in transgenic tobacco was under the control of CaMV 35S promoter. In;transgenic leaf tissues, CBHI activity up to 66.1 mu mol/h/g total protein was observed. In transgenic calli, the highest CBHI activity was 83.6 mu mol h/g total protein. Protein immunoblot analysis confirms the presence of CBHI enzyme in both transgenic calli and leaf tissues. CBHI expression levels accounted for about 0.11% and 0.082% of total protein in transgenic leaf tissues and calli, respectively. Furthermore, expression of CBHI gene did not affect normal growth and development of transgenic plants.