Electron dense bodies and vesicles were increased in undifferentiated crypt cells and differentiated colonocytes in the rectal mucosa of healthy volunteers in southern India. In addition, in the surface colonocytes lysosomes were increased, the cells were shorter with short irregularly grouped microvilli, there was evidence of cell immaturity and a high prevalence of spiral organism infestation. There was also alterations in goblet cell mucus granules, a reticulohistiocytic response in the subluminal lamina propria and residual evidence of vascular damage. These alterations indicate a non-specific response to mucosal injury. Such changes have not been observed in the rectal mucosa of temperate zone controls and could be designated tropical colonopathy.