An isoenzyme of glucosidase- isolated from sweet almond emulsin - and composed of a β-D-glucosidase, a β-D-galactosidase and a β-D-fucosidase, has been shown to possess β-D-xylosidase activity, as well. On the basis of the following results it has been concluded that the β-D-glucosidase and β-D-galactosidase activities reside in one catalytic site, but there are two kinetically distinst binding sites in the active center: 1./D-Glucono-1,5-lactone is shown to excert competitive inhibition on the hydrolysis of β-D-glucopyranoside and non-competitive inhibition on the hydrolysis of β-D-galactopyranoside. 2./ D-galactono-1,5-lactone competitively inhibits the hydrolysis of β-D-galactopyranoside, but possesses non-competitive inhibition on the hydrolysis of β-D-glucopyranoside. 3./ When the enzyme is incubated with two p-nitrophenyl glycoside substrates at or above their respective Km values, the rate of p-nitrophenol formation is not additive but rather it is equal to the value calculated from the individual Km values and relative maximum rates.