Ultrastructural and cytochemical techniques showed that there was a marked difference in the structure and cytochemistry of the hyphae from old and young parts of mycelial strands produced by the mycorrhizas of Pinus radiata D. Don. The older strands were differentiated into an outer region of thick walled hyphae and inner region of thin walled hyphae of two types. Histochemical tests for carbohydrates were used to distinguish between gels produced by the micro-organisms associated with the strands and those by the fungal hyphae themselves, and to distinguish between the polysaccharides of the mycelial gel and those of the layers in the cell walls of the hyphae. Carbohydrate distribution in and on the strands is discussed in relation to the breakdown of litter and to the stability of soil crumbs under pine forests.