The 6.3 kb plasmid pCRB1 containing the eglS gene from Streptomyces rochei, coding for a beta-1,4 glucanase, was constructed in Bacillus subtilis by using the plasmid vector pIL253 with subsequent activity of the cellulase activity. Strictly anaerobic cellulolytic and xylanolytic strains of Ruminococcus albus, isolated from the rumen of cows and water buffaloes, were used for transformation experiments. The plasmid pCRB1 was introduced by means of electroporation into five freshly isolated strains of R. albus, with frequencies ranging from 10(3) to 10(4)/mg of plasmid DNA. Northern analysis demonstrated the expression of the eglS gene in R. albus. All the strains harbouring the heterologous cellulase gene showed an increase of the secreted cellulase activity.