The crystallization and melting of un vulcanized natural rubber in the unstretched state have been investigated at different temperatures. Change of volume has been used as a quantitative measure of the extent of crystallization, and mercury-filled dilatometers containing the rubber have been u sed for the volume measurements. Crystallization was observed to occur at temperatures between -50° and I Presented at the meeting of the Division of High Polymer Physics of the American Physical Society held at