A DNA genomic library constructed from Bacillus stearothermophilus, a gram-positive, facultative thermophilic aerobe that secretes a thermostable -mannanase, was screened for mannan hydrolytic activity. Recombinant -mannanase activity was detected on the basis of the clearing of halos around Escherichia coli colonies grown on a dye-labelled substrate, Remazol brilliant blue-locust bean gum. The nucleotide sequence of the mannanase gene, manF, corresponded to an open reading frame of 2,085 bp that codes for a 32-amino-acid signal peptide and a mature protein with a molecular mass of 76,089 Da. From sequence analysis, ManF belongs to glycosyl hydrolase family 5 and exhibits higher similarity to eukaryotic than to bacterial mannanases. The manF coding sequence was subcloned into the pH6EX3 expression plasmid and expressed in E. coli as a recombinant fusion protein containing a hexahistidine N-terminal sequence. The fusion protein has thermostability similar to the native enzyme and was purified by Ni²⁺ affinity chromatography. The values for the kinetic parameters V_max and K_m were 384 U/mg and 2.4 mg/ml, respectively, for the recombinant mannanase and were comparable to those of the native enzyme.