The gene (egI) encoding a cellulase (EgI) from Ruminococcus albus was modified for production of the three forms of a cellulase : the pre-form having a signal sequence region, the mature form lacking the signal sequence region and a truncated form, which contained neither the signal sequence nor the N-terminal 15 amino acid residues. These modified genes were expressed in tobacco suspension cells (BY-2) under the control of cauliflower mosaic virus 35S promoter. Among the transformants harboring each type of the gene, cells having the gene encoding the truncated egI showed the strongest cellulase activity. In this transformant, 95% of the cellulase activity was located in cytoplasm and the total cellulase activity was approximately 30 times stronger than the endogenous cellulase activity in the non-transformed tobacco cells. The growth of the transformants harboring the gene encoding the trucated EgI was negligibly affected by the high level of expression of the truncated EgI.