Washed mycelia of Aspergillus nidulans FGSC159 were incubated in CMC minimal liquid medium and the culture filtrate which contained induced extracellular cellulase was fractionated by a three-step procedure including chromatography on Bio-Gel P-150, chromatography on DEAE-Sephadex A-50 and chromatography on Sephadex G-100. Three CMCase components ; F-I-Ia, F-I-Ib and F-II-Ia were prepared. No enzyme activity toward avicel could be detected in these components. Similarly, there was no ${eta}-glucosidase$ activity. pH-optima of the three components were all 5.0 in acetate buffer. Temperature-optima for the activities of F-I-Ia, F-Ib and F-II-Ia were $45^{circ}C,;40^{circ}C;and;50^{circ}C$, respectively. F-II-Ia was shown to be more thermostable than the other two components. F-II-Ia was proved to have quite a different substrate specificity and action property and action property from those of F-I-Ia and F-I-Ib by product analysis on liquid chromatography.