The composition and ultrastructure of spore walls of Agaricus bisporus and Agaricus campestris were compared by chemical and enzymatic assays and electron microscopy. Polymers of N-acetylglucosamine (chitin) and glucosamine (chitosan) were the major carbohydrate wall components of both species. The chitin to chitosan ratio in the spore walls of A. bisporus was about 0.38, while in A. campestris it was about 2.8. These polymers were associated with proteins, lipids (readily extractable and bound), melanin, and a low content of β-glucan. The quantity of protein, total lipid, melanin, or β-glucan was similar in spore walls of both species. With the electron microscope, the spore walls appeared as two well-defined layers which may correspond to microfibrils of chitin and chitosan in which the electron-dense melanin was located in the external layer.