Bacillus sp. JB-99, when grown in a chemically defined medium containing lactose as a carbon source, yielded 3,860 U/ml extracellular $eta$-mannanase, which was high compared to other examined carbon sources. Among the nitrogen sources, yeast extract enhanced the enzyme activity. The enzyme production was growth-associated. The enzyme was optimally active at $65^{circ}C$, pH 10, and had a half-life of 190 min at $65^{circ}C$. N-Bromosuccinamide and $AgNO_3,;CuSO_4$, and $HgCl_2$ strongly inhibited the enzyme, whereas $Ca^{2+}$ stimulated the enzyme activity. The $alpha$-galactosidase enzyme production was not found in any of the enzyme assays.