In order to confirm the chemical structure of the mannan of Pichia pastoris IFO 0948 strain, which was assumed to possess a highly branched comb-like structure, a sequential degradation procedure involving partial acid hydrolysis followed by acetolysis under mild conditions was employed. This procedure can widely be adopted as a method for determining the structural shape of fungal mannans possessing a highly branched structure, either comb-like or tree-like, without application of enzymolytic degradation.