Greenhouse-grown l-year-old sweet orange trees [Citrus sinensis (L.) Osbeck cv. Valencia] on cold-hardy trifoliate orange [Poncirus trifoliata (L.) Raf.] and cold-sensitive citron (C. medica L.) rootstocks were exposed to cold-acclimation conditions and freeze-tested at -6.7C for 4 hours in a temperature-programed walk-in freezer room. Nonhardened trees generally did not survive the freeze, whereas cold-hardened trees survived with no wood kill on either rootstock. Essentially, all leaves died or abscised during the subsequent 5 weeks in the greenhouse. Freeze survival did not separate rootstocks nor did supercooling in separate trials where Yalencia' wood reached –8.8C before apparent nucleation. Increases in concentration of carbohydrates and proline and decreases in water content in Yalencia' leaves during cold hardening were generally associated with increased freeze tolerance. Other tests, that matched 9-month-old seedlings of citron with trifoliate orange rootstock, showed clear differences in the superior cold acclimation of trifoliate orange over citron, which, however, performed better than expected.