We have cloned the Bacillus cellulyticus K-12 avicelase(Avi, E.G.3.2.1.4) gene(ace A) in E. coli. This was accompanied by using the vector pT7T3U19 and Hind Ⅲ -HindⅢ libraries of Bacillus cellulyticus K-12 chromosomal inserts created in E. coli. The libraries were screened for the expression of avicelase by monitoring the immunoreaction of the anti-avicelase(immunoscreening). Positive clones(Ac-3, Ac-5, and Ac-7) contained the identical 3.5kb HindⅢ fragment as determined by restriction mapping and Southern hybridization, and expressed avicelase efficiently and constituvely using its own promoter in the heterologous host. From the immunoblotting analysis, a polypeptide which showed a CMCase activity with an Mr of 54000 was detected.