In a first aspect, the present invention relates to a method for isolating cell wall derivatives from fungal or yeast biomass. According to this method, chitin polymers or chitin-glucan copolymers can be obtained. In another aspect, the invention relates to a method for preparing chitosan from chitin. The invention further relates to chitin polymers, chitin-glucan polymers and chitosan polymers obtainable by the methods according to the invention. Moreover, the invention relates to the use of chitin polymers, chitin-glucan copolymers or chitosan polymers obtainable by the method according to the present invention in medical, pharmaceutical, agricultural, nutraceutical, food, textile, cosmetic, industrial and/or environmental applications.