Porphyra umbilicalis is a monostromatic thallus consisting of a single layer of cells with thick cell walls, embedded in a mucilaginous intercellular matrix and sandwiched between the tough outer 'cuticles'. It has been reported in the past that the cell wall consists of β-1,3 xylan, the 'cuticle', a β-1,4 mannan and the intercellular matrix, a sulphated D-L-galactan. These studies had been made on materials extracted by rather drastic chemical treatments. An attempt to separate these regions in a pure and native form using biochemically mild techniques was met with success. Chemical, physical and biochemical studies show these to be complex structures whose formation and function are biologically controlled. All three extracellular regions are found to contain proteins in addition to polysaccharides which form their major constituent. The protein component contains all the common amino acids with relatively higher amounts of serine and threonine. Cysteine was found to be high in the 'cuticle' and in the cell wall. Hydroxyproline was absent from all three regions studied. Mannose and glucose were found to form the major constituents of the polysaccharide component of the 'cuticle', xylose and glucose with small amounts of uronic acid in that of the cell wall and galactose, 6-0-methyl galactose, 3,6-anhydrogalactose and sulphate half ester in that of the intercellular matrix. The presence of an enzyme causing methylation of galactose units of the intercellular matrix is reported. This enzyme appears to be playing an important role in bringing about fine structure modifications of the matrix polysaccharide, thereby preserving the structure and conformation for its biological function. Also an enzyme activity, degrading the matrix polysaccharide, is demonstrated which suggests that the production and functioning of this extracellular region is biologically controlled.