An intracellular beta-galactosidase from a thermoacidophilic A licyclobacillus acidocaldarius subsp. rittmannii was purified by procedures including precipitation with ammonium sulphate, gel permeation, ion-exchange and affinity chromatography and finally by preparative electrophoresis and some properties of the purified enzyme were determined. The homogenous enzyme had a specific activity of 113 U/mg protein, with a fold purification of 163 and a yield of 8%. The Km and kcat values for ONPG were determined as 8.9 mM and 1074 min(-1), respectively in the purified beta-galactosidase from A. acidocaldarius subspecies rittmannii. The bacteria produce thermostable P-galactosidase (EC 3.2.1.23) activity, which exhibits its optimum at the neutral pH region. The pH and temperature optima for the purified enzyme are 6.0 and 65 degrees C, respectively (10 min assay). beta-Galactosidase specific activities of crude extracts obtained from bacterial cells grown in the presence and absence of lactose over a period of time (6-40 h) showed that beta-galactosidase synthesis seems to be constitutive and increases by increasing time up to 40 h of cultivation. beta-Galactosidase activity in the bacteria growing on the medium without lactose was 0.4 U/mg protein and increased up to 0.6 U/mg protein in the cells growing on the medium with lactose at 24 h (an increase by about 33% of its constitutive value), while it was 0.072 and 0.48 U/mg protein, respectively at 12 h (an increase by about 85% of its constitutive value). IPTG was also found to increase beta-galactosidase activity over a short period of time. The molecular mass of the purified enzyme determined by gel filtration on FPLC and SDS-PAGE was 165 and 76 kDa, respectively. The beta-galactosidase from Alicyclobacillus acidocaldarius subsp. rittmannii is most likely to belong to the glycoside hydrolyse family 42. (C) 2006 Elsevier Inc. All rights reserved.