Examination of the carbohydrates of cell walls prepared from tuber discs of a susceptible variety of potato showed an increase, with time, in all the polysaccharide fractions in control discs, but a slower increase in the pectic fraction and a more rapid increase in the extraction residue to discs infected with Phytophthora infestans. These differences were related to the monosaccharide composition of hydrolysates; there was no increase in galactose, found predominantly in the pectic fraction, but a rapid increase in glucose which is confined almost exclusively to the extraction residue. Part of the increased glucose was due to an accumulation of hyphal wall of P. infestans which contains mainly an alkali-insoluble glucan. Galactanase activity, which was demonstrated in infected discs, could account for the divergence of galactose content from that of the controls. There was an enhanced accumulation of a lignin-like polymer associated with the cell-wall fraction of infected discs.