The tibio-fibular interosseous membrane was studied in the scanning electron microscope. It is made up of two fibrous networks, a major one composed of large diameter fibres aggregated into bundles and a fine filamentous network, without a preferred orientation, between these main fibre bundles. Mechanical studies have shown that the interosseous membrane is stronger, but less extensible, parallel to the direction of the main fibre bundles which, therefore, probably have a load bearing function.