Highly polymerized cellulosic substances induced marked formation of cellulase [EC 3. 2. 1. 4] in Pseudomonas fluorescens var. cellulosa and more than 90 per cent of the enzyme was released into the culture medium. The extracellular cellulase seemed to consist of two components. Sophorose had a similar inductive effect, while gentiobiose had a similar but weaker effect. In contrast, cultures on 0.5 per cent cellobiose or cellooligosaccharides synthesized only a small amount of cellulase while more than 90 per cent of the enzyme was in a bound form in the cells. This cellulase seemed to consist of a single component, and its electrophoretic mobility differed from those of the extracellular components. In cultures with controlled supply of various C-sources, formation of extracellular cellulase was enhanced to a comparable extent to that in cultures containing cellulose or sophorose, but formation of cell-bound cellulase did not increase significantly. Paucity of absorbable cellulolytic products in the culture medium appears to result in enhanced extracellular cellulase formation.