7 Fun facts about fungi! Fungus, plural fungi, is any member of the group of about 144,000 known eukaryotic organisms of the kingdom Fungi, which includes yeasts, rusts, smuts, mildews, molds, and mushrooms.  They are found in just about any habitat but most live on the land, mainly in soil or on plant material, and they come in all shapes and sizes.  Some also live in the sea or in fresh water.