The function of the anterior tibiofibular ligament, of the syndesmosis between the distal parts of the tibia and fibula, and of the posterior tibiofibular ligament was elucidated by tracing mobility patterns before and after successive ligament transection in varying sequence. Eighteen osteoligamentous preparations were studied. Mobility was only minimally influenced by isolated cutting of the anterior tibiofibular ligament, and even complete cutting of the distal tibiofibular ligaments involved only minor abnormality in motion. However, external rotation was greatly increased by further cutting of the anterior part of the deltoid ligament or of the posterior talofibular ligament. This indicates that isolated injury of the anterior tibiofibular ligament must be rare, and total rupture of the distal tibiofibular structures is presumably as a rule combined with lesions of the anterior part of the deltoid ligament and/or of the posterior talofibular ligament