Forty-eight yeast strains belonging to the genera Candida, Debaryomyces, Kluyveromyces and Pichia (obtained from the ARS Culture Collection, Peoria, IL) were screened for production of extracellular glucose tolerant and thermophilic beta-glucosidase activity using p-nitrophenyl-beta-D-glucoside as substrate. Enzymes from 15 yeast strains showed very high glucose tolerance (< 50% inhibition at 30%, w/v glucose). The optimal temperatures and pH for these beta-glucosidase activities varied from 30 to 65 degrees C and pH 4.5 to 6.5. The beta-glucosidases from all these yeast strains hydrolyzed cellobiose.