The distribution of cellulase activity in the solid and liquid fractions of rumen contents collected from a fistulated steer was studied. The specific activity of cellulase in the clear supernatant of rumen fluid and in the sonicated cells suspended from the liquid fraction was 0 and 0.75 × 10−3 units/mg protein respectively against carboxymethylcellulose (CMC). On the other hand, the specific activity of the fraction obtained by ultrasonic disruption of organisms released from the solid portion of rumen contents by gentle agitation was 0.22 × 10−3, 0.19 × 10−3, and 1.10 × 10−3 units/mg protein against solka floc, avicel, and CMC respectively. Addition of autoclaved rumen fluid to the assay system resulted in a decrease in the rate of production of reducing sugars, indicating the presence of inhibitors. The cellulase exhibited a maximum activity at pH 5.5. The addition of cobalt chloride and L-cysteine resulted in an increase in cellulase activity whereas the addition of urea, D-gluconic acid lactone, sodium sulfide, and ferric chloride was inhibitory.In cellulose enrichment cultures obtained from 10−8 dilution of rumen liquor, the specific activity of cellulase in the supernatant was always less than in the fraction obtained by sonic disruption of cells attached to the filter paper. The results are discussed in relation to the localization of cellulases in rumen microorganisms.