The main storage polysaccharide used by fungi is glycogen.
Glycogen is a glucose polymer that stores chemical energy inside fungal cells. Its many branches provide numerous ends where enzymes can add or remove glucose quickly, making it useful when a fungus needs energy for growth, spore production, or metabolism.
This storage strategy helps distinguish fungi from plants. Plants commonly store glucose as starch, whereas fungi—like animals—use glycogen. Fungal cells also contain other soluble carbohydrates, including trehalose and sugar alcohols such as mannitol, but these do not replace glycogen as the characteristic storage polysaccharide.
Glycogen is structurally similar to amylopectin, a component of starch, but is more extensively branched and compact. Another common mistake is confusing storage material with cell-wall material: fungal walls are strengthened mainly by chitin, which is structural rather than an energy reserve.