We report- the partial characterization of a beta-glucosidase produced during growth of the thermotolerant yeast, K. marxianus IMB3 on lactose-containing media at 45 degrees C. The enzyme had Km values of 1.1 mM and 14.8 mM, for the substrates p-nitrophenyl-beta-D-glucoside and cellobiose, respectively. The enzyme had a pH optimum of 5.5 and was optimally active at 50 degrees C. It was stable up to 125 hours at 25 degrees C and 35 degrees, with half-lives of 45 hours and 2 hours at 45 degrees C and 50 degrees C, respectively. inactivated by Hg2+. Ethanol concentrations [1-10% (v/v)] had little effect an activity. Glucose (20 mM) caused inhibition when p-nitrophenyl-beta-D-glucoside was used as substrate, whereas lactose at similar concentrations had no effect.