Infection of barley leaves by the fungus Rhynchosporium secalis increases the degree to which stomata open in the light. Opening is enhanced at CO2, levels between 0 and 4000 parts/106. The ability of stomata to close in the dark, as normal, is retained until an advanced stage of tissue necrosis is reached. Increased stomatal opening is confined to those areas of the leaf which have been colonized by the fungus. Abnormal stomatal behaviour results from the loss of osmotically active substances from the epidermis of diseased leaves with a consequent alteration of the turgor relations between guard cells and their surrounding epidermal cells.