The thermal and proteolytic stability of free and immobilized beta-D-glucosidase, isolated from Pseudomonas pickettii and Aspergillus niger, were determined. The optimal temperatures of soluble and entrapped beta-glucosidase extracted from P. pickettii were 40 and 50 degrees C, respectively. In contrast, the optimal temperature of enzyme isolated from A. niger remained unaltered (60 degrees C). Free and immobilized A. niger beta-glucosidase showed an unusual discontinuity around 40 degrees C in the Arrhenius plot, suggesting that the enzyme could exist in two (or more) interconvertible forms with different activation energies. The polymeric network influenced the reactivity of both fungal and bacterial a-glucosidases since their E-a values changed with respect to their soluble counterparts. Nevertheless, both the thermal stability and the resistance to proteolysis were apparently related to the origin (bacterial or fungal) and location (intracellular or exocellular) of the enzyme. The half-lives of soluble and immobilized beta-glucosidases at six different temperatures were also calculated. The properties assayed were compared critically with those reported by other authors.