A beta-1,4-mannanase, termed AoMan134A, that belongs to the GH 134 family was identified in the filamentous fungus Aspergillus oryzae. Recombinant AoMan134A was expressed in Pichia pastoris, and the purified enzyme produced mannobiose, mannotriose, mannotetraose, and mannopentaose from galactose-free beta-mannan, with mannotriose being the predominant reaction product. The catalytic efficiency (k (cat)/K (m) ) of AoMan134A was 6.8-fold higher toward galactomannan from locust bean gum, than toward galactomannan from guar gum, but similar toward galactomannan from locust bean gum and glucomannan from konjac flour. After incubation at 70A degrees C for 120 min, the activity of AoMan134A toward glucomannan decreased to 50% of the maximal activity at 30A degrees C. AoMan134A retained 50% of its beta-1,4-mannanase activity after heating at 90A degrees C for 30 min, indicating that AoMan134A is thermostable. Furthermore, AoMan134A was stable within a neutral-to-alkaline pH range, as well as exhibiting stability in the presence of a range of organic solvents, detergents, and metal ions. These findings suggest that AoMan134A could be useful in a diverse range of industries where conversion of beta-mannans is of prime importance.