New extracellular endoglucanases, designated RCE1 and RCE2, produced by Rhizopus oryzae isolated from the soil, were purified to apparent homogeneity from the culture supernatant. The molecular mass of RCE1 and that of RCE2 were found to be 41 kDa and 61 kDa, respectively. The N-terminal amino acid sequences of RCE1 and RCE2 showed high homology with those of the family I cellulose-binding domains. Internal amino acid sequences of RCE1 and RCE2 showed homology with that of the catalytic domain of EGV from Humicola insolens belonging to family 45 endoglucanase. The cellooligosaccharide hydrolysis patterns of RCE1 and RCE2 were similar to that of EGV from H. insolens. These results indicate that RCE1 and RCE2 are family 45 endoglucanases having a cellulose binding domain at their N-terminus. RCE1 and RCE2 hydrolyzed carboxymethylcellulose (CMC), insoluble cellooligosaccharide (G33), cellohexaose, and cellopenpaose, but not Avicel, xylan, galactan, arabinan, mannan, or laminarin. The CMCase activity of both enzymes was inhibited by Cu2+, Zn2+, Co2+, and Pb2+. The optimum pH for the CMCase activity of both enzymes was found to be between pH value 5.0 and 6.0, and the optimum temperature was 55degreesC. the lowest among the family 45 endoglucanases. These results indicate that RCE1 and RCE2 represent a new type of endoglucanases having the lowest optimum temperature among the family 45 endoglucanases.