The structure of the lower oesophageal sphincter (LOS) of the cat was investigated in comparison with that of the oesophagus. The following two main results were collected: (1) The muscularis mucosae of the lower oesophageal sphincter is much thicker than at oesophageal level (221 µm as compared with 17.8 µm on average). This thickening is particularly marked at the most prominent foldings of the mucosa. (2) Numerous annulo-spiral thin elastic fibres were found, in the circular layer of the LOS. These structures will coil spirally around muscular bundles and then wrap themselves around adjacent bundles so that they present an oblique orientation with regard to muscular fibres. Therefore it is concluded that in this species, the LOS is morphologically differentiated from the oesophagu