The structure of the tapetum of the sheep's eye has been examined by transmission electron microscopy. It consists of regularly arranged layers of collagen fibrils, estimated from the hydroxyproline content to make up about 60 percent of the dry weight of the tissue. There are several hundred of these layers in the whole depth, a majority being oriented in the same direction. The collagen is associated with strands of microfibrillar material as well as with cells of both fibrocyte and smooth muscle types.