During growth on 0.2% (w/v) microcrystalline cellulose, Bacteroides succinogenes S85 produces endoglucanase activity which can be separated by centrifugation into sedimentable and nonsedimentable fractions. The sedimentable activity, after solubilization with Triton X-100, was resolved into four components by ion-exchange chromatography and these were further fractionated by nondenaturing polyacrylamide gel electrophoresis (PAGE). The nonsedimentable activity contained three enzymic components as determined by gel filtration. Like the preparations derived from the sedimentable fraction, these components yielded a multiplicity of endoglucanases when electrophoresed under nondenaturing conditions. The fractions obtained by ion-exchange chromatography and by gel filtration were assayed for endoglucanase activity by both viscometric assays and reducing sugar production using carboxymethylcellulose as the substrate. Plots of the fluidity change in the enzyme–substrate preparation in relation to reducing sugar production revealed the presence of two distinct groups of endoglucanases differing in catalytic activity. Two of the components from the nonsedimentable fraction had more exoglucanase-like activity than either the third nonsedimentable fraction or any of the four fractions derived from the sedimentable material. These two enzymes could be further differentiated on the basis of glucose production from microcrystalline cellulose and by their relative activity toward p-nitrophenyl cellobioside, a chromogenic substrate.