1. Woody outdoor seedlings of guayule not yet unacclimated to low temperatures survived only slightly lower temperatures than did very succulent greenhouse plants of the same age. 2. The rate of cooling, preceding an extended exposure to low temperatures, did not appreciably alter the resistance of the plant to frost injury. 3. Unhardened seedlings of four varieties withstood prolonged exposure to a temperature of 25⚬ but were severely injured as the result of an 8-10-hour exposure to 22⚬ F. 4. In most instances, the roots were severely injured by prolonged exposure to soil temperatures of 26⚬-28⚬ F. As an exception, roots of plants of selection A-5058 survived a short exposure (less than 3 hours) to a soil temperature of 16⚬ F. 5. Stems and leaves of transplants were readily acclimated by means of low temperature hardening treatments, so that they withstood repeated exposure to temperatures ranging 5⚬-10⚬ above zero. Stems of hardened plants of selection A-5058 withstood a 3-hour exposure to a temperatur...