A purified wall fraction was prepared from the mycelium of Agaricus bisporus. The isolated wall consisted of 43",, (wiw) chitin, 1 4 "~) KOH-soluble glucan, 27Oh /I-glucan, 1 6 O , protein and ~-5~) , , lipid. Traces of mannose and xylose were detected by gas chromatography. The architecture of the wall was investigated with sequential enzyme digestion and electron microscopy. The outer wall surface is covered by a mucilage that is removcd by the isolation procedure. The wall fraction could be completely degraded by extraction in warm I M-KOH followed by digestion with a mixed /,'-glucanase and then chitinase. The outer layer of KOHsoluble glucan is amorphous and or variable thickness. Incubation with glucanase did not change the dimensions of the wall but was necessary before the inner wall was susceptible to attack by chitinase, indicating that />'-glucan does not constitute a separate wall layer but is a matrix associated with the fibrillar chitin. The inner layer presents an even, compact. fibrillar side as the inside surface of the wall, but is looser and uneven outwardly where it interdigitates with the irregular inner surface of the KOH-soluble glucan. Silver hexamide staining showed cystine-containing protein throughout the wall.The fruit body of the Agaricales is massive, yet mainly consists of relatively undifferentiated hyphal elements similar to the vegetative mycelium, and thus provides an interesting system for studying the mechanisms of hyphal aggregation i n morphogenesis. A number of striking correlations between fungal cell form and wall composition, particularly in such phenomena as hyphal-yeast cell type dimorphism, emphasize the importance that wall composition may have in determining dif'ferentiation and rnorphogenesis (Smith & Galbraith, 1971). The composition and architecture of the hyphal wall is now known in a variety of fungi, but a more or less complete picture is available for only one near relative of the mushrooms, Schizopliyllrrm co~iiniuno, a member of the Aphyllophorales. We present here a study of the hyphal architecture 01' the common mushroom, Agirricus bisyorus, and compare and relate it to S. coyIztizunc and the existing fragmentary picture in the Agaricales itself.