Glucan, mannan, chitin, protein, phosphate and lipid are found in the walls of all brewing yeasts so far examined although the contents and composition of each varies, as does the content of wall in the cell. Both glucan and mannan have branched structures and are linked to protein. Chitin is probably also bound to protein and phosphate to mannan. Some of the enzymic activities present in the wall are associated with the mannan‐protein fraction. This fraction is located on the outer surface of the wall and appears to play a part in the appearance of flocculence in brewing yeast during fermentation.