Significant differences in chemical composition and structure of wall polysaccharides were found in Agaricus bisporus vegetative mycelium in comparison with the fruit body mycelium. Chemical fractionation of the walls demonstrated distinct percentages of the successively solubilized polysaccharides in both walls, and different proportions of their respective sugar monomers. Permethylation of isolated fractions also showed striking differences relative to the sugar linkages between the two types of walls studied. Furthermore, infrared spectrophotometry exhibited a distinct polysaccharidic configuration in several fractions. All the described differences were correlated with the transition of vegetative mycelial walls to the fruiting stage of differentiation.