In this study the mannan-degrading system of the Gram positive bacterium Cellulomonas fimi was characterised. C. fimi can degrade different forms of mannan and can use the degradation products as carbon and energy source. This study focuses on the galactomannan-degrading system which was found to be composed of one secreted endo- 1,4- β -mannanase (Man26A), one intracellular l,4-β-mannosidase (Man2A) and one intracellular 1,6-α-galactosidase. The genes encoding Man26A and Man2A have been isolated and sequenced, and the enzyme activities were investigated. The endo-l,4- β -mannanase (Man26A) has a multidomain structure and comprises a family 26 catalytic domain, a mannan-binding domain (MBD), a S-layer homology domain (SLH domain) and a domain of yet unknown function. Mannanase activity was detected on the cell surface and in the culture supernatant. It is believed that the SLH domain mediates transient binding of Man26A to the cell surface and the MBD mediates binding to the substrate. Strong binding of the MBD to soluble mannan was detected and its potential as an affinity tag for protein purification in aqueous 2-phase systems was tested. The 1,4- β -mannosidase (Man2A), cleaves β -1,4 mannosidic linkages with net retention of the anomeric configuration. Man2A was transformed into the glycosynthase Man2A E519A by mutating the catalytic nucleophile, E519 to alanine. Glycosynthases are retaining glycosidases without hydrolytic but with synthetic activity. Using α-mannosyl fluoride as donor and p-nitrophenyl sugars as acceptors, the glycosynthase Man2A E519A catalyzed the synthesis of β -1,4 and β -1,3 mannosidic linkages. In this study the biology of mannan-degradation by C. fimi was investigated and the biotechnological potential of its components was explored.