We have used the endo-β-1,4-mannanase from Trichoderma reesei (Tr Man5A), the endo-β-1,4-mannanase from Aspergillus niger (An Man5A) and the exo-β-1,4-mannosidase from A. niger (An Mnd2A) to follow the enzymatic degradation of mannan I and II crystals. The degradation process was studied by transmission electron microscopy and also followed by analysis of the released soluble reducing sugars. The mannan crystals were degraded by the endo-β-1,4-mannanases and to a lesser extent by the exo-β-1,4-mannosidase. The observed hydrolysis pattern on mannan I crystals is fully consistent with the current view of the molecular structure of these crystals. The molecular organization of the mannan chains in mannan II crystals is less clear and the digestion results give some further information about the ultrastructure of mannan II. In addition, insight is provided into the mode of the enzymatic attack on the crystals of mannan I and mannan II.