The present invention relates to mannanases comprising e.g., a sequence of amino acids 32 330 of SEQ ID NO: 2 or their homologues may be derived from e.g., Bacillus sp. I633, or may be encoded by polynucleotide molecules comprising a sequence of nucleotides 94 990 of SEQ ID NO: 1 from, polynucleotide molecules that encode a polypeptide that is at least 65% identical to the sequence of amino acids 32 330 of SEQ ID NO: 2, or degenerate nucleotide sequences thereof. The mannanases are alkaline and are useful e.g. in cleaning compositions, in a fracturing fluid useful to fracture a subterranean formation, for modifying plant material, and for treatment of cellulosic fibers.