Potato mop-top virus, one of the most commonly occurring viruses in virus tested stocks of seed potatoes in the United Kingdom induces four different haulm symptoms which are both climate and variety dependent. Ultrastructural examination showed that the aucuba symptom recognizable as bright yellow patches on the leaves, and the mop-top symptom characterised by the dwarfed and bunched habit of the plants, both contained in their leaf cells, tufts and clusters of microtubule-like elements, although the other ultrastructural features associated with each symptom were quite different. These mop-top tubules occurred in the cytoplasm, between the cell wall and the plasma membrane, and in the vacuole, and have been demonstrated in every cell type.