A 1 250 bp cDNA fragment encoding β-mannanase, named MaMAN, was cloned from banana (Musa spp cv. Baxi) fruit using degenerate primers designed with reference to the conserved nucleic acid sequences of known β-mannanase genes by RT-PCR. Sequence analysis showed that MaMAN cDNA encompassed a 1 085 bp open-reading frame (ORF), encoding a predicted polypeptide of 395 amino acids. Alignment of the deduced amino acid sequence of MaMAN and other putative β-mannanases showed that MaMAN has an identity of 86, 70, 69, 54, and 57%, respectively, to β-mannanases from tomato, lettuce, arabidopsis, carrot and oryza sativa. The catalytic residues: Asn203, Glu204, Glu318 and the active site residues: Arg86, His277, Tyr279, and Trp360, which were strictly conserved in the glycoside hydrolase family 5 to which all 3-mannanases belonged, were found in MaMAN. Semi-quantitative RT-PCR revealed that the level of MaMAN transcript in the pulp increased during banana fruit ripening, suggesting that MaMAN was likely to be involved highly in banana fruit softening.