Endoglucanase ofRuminococcus sp. is composed of seven active protein components when chromatographed on an ion exchange column (Q-Sepharose). Component I (endoglucanase A) did not bind to the column and was purified to homogeneity by molecular sieve chromatography. It had a mol. wt. of 22 000. Component II was fractionated into two active protein peaks (endoglucanase B and C) having mol. wt. of 225 000 and 10 000. The endoglucanase A had high affinity for CMC (Km 8 mg/ml). The temperature optimum of all three endoglucanase was between 40–45°C. The gene encoding for endolucanase activity was cloned inE. coli HB101 with pBR322. A 4.3 kilobaseBamH1 fragment encoding endoglucanase was hybridized toRuminococcus chromosomal DNA.