The ultrastructure of cotton leaves, exhibiting reddening as symptom of physiological disorder, was examined by means of transmission electron microscopy. Osmiophilisation of the membrane compartment was established. Massive agglomerations on the tonoplast in the vacuole of cells under the adaxial epidermis were observed, and were referred to as electron-dense osmiophilic substance, most probably of anthocyanin nature. In chloroplast stroma a zone of low electron density enclosing numerous osmiophilic aggregations of unclear chemical character was differentiated. Fragmentation and severe destruction of thylakoids in chloroplasts of reddening cotton leaves was not detected.