Trichoderma reesei FTKO-39 grown at 35 degrees C for 5 d on wheat bran supplemented with MgCl2 and lactose as the carbon source produced two isozymes of beta-galactosidase: BGT I and BGT II. These isozymes were partially purified on a DEAE-Trisacryl column. Both BGT I and BGT II fractions exhibited optimum activity at 65 degrees C, but the pH optima were 4.0 and 6.5, respectively. The isozymes also showed similar thermal stability. However, BGT I was more stable than BGT II in a pH range of 3.0-10.0. At least two different beta-galactosidases are produced by T. reesei, as revealed by the two bands seen on a 6% polyacrylamide gel stained for activity.