When fibrovascular sap extracts of sugar-cane plants affected by the ratoon stunting disease (RSD) were centrifuged and the resuspended pellets negatively stained and examined in an electron microscope, cells of a small bacterium were always observed. The bacterium could be distinguished readily from other bacteria present by its small size (usually 1.0–2.5 µm long by 0.15–0.32 µm wide), the coryneform (club-shaped) morphology of some cells, and its permeability to negative stain revealing a thin cell wall surrounding a cytoplasmic membrane and coiled mesosomes. Since the small bacterium was never observed in fibrovascular extracts of RSD-free sugar-cane plants, it is a possible causal agent of RSD.