Ice nuclei active at approximately -2°C and intrinsic to woody tissues of Prunus spp. were shown to have properties distinct from bacterial ice nuclei. Soaking 5-centimeter peach stem sections in water for 4 hours lowered the mean ice nucleation temperature to below -4°C, nearly 2°C lower than stems inoculated with ice nucleation-active Pseudomonas syringae strain B301D. Ice nucleation activity in peach was fully restored by air-drying woody stem sections for a few hours. The ice nuclei in woody tissue were inactivated between 40 and 50°C, but unaffected by treatment with bacterial ice nucleation inhibitors (i.e. NaOCI, tartaric acid, Triton XQS-20), sulfhydryl reagents (i.e. p-hydroxymercuribenzoate and iodine) and Pronase. Ice nuclei could not be dislodged from stems by sonication and were shown to be equally distributed in peach bud and internodal stem tissue on a per unit mass basis; outer and inner stem tissues were also indistinguishable in ice nucleation activity. Development of ice nuclei in immature peach and sweet cherry stems did not occur until midsummer and their formation was essentially complete by late August. Once formed the ice nuclei intrinsic to woody stems were stable and unaffected by seasonal changes in growth. The apparent physiological function of the ice nuclei is discussed in relation to supercooling and mechanisms of cold hardiness in Prunus spp.Frost tender plants avoid intracellular ice formation and concomitant lethal injury because of the capacity of water in plant cells to supercool. Although pure water supercools to approximately -40°C before homogeneous ice nucleation occurs, ice generally forms at higher temperatures because exogenous ice nuclei order water molecules into a configuration conducive to growth of ice crystals (23, 29). An ice nucleus is active at a discrete threshold temperature, and only a few substances, primarily crystalline forms of organic compounds, show ice nucleation activity at temperatures above -5°C (14). Accordingly, water within the tissues of annual plants, such as beans, tomatoes and maize, has the capacity to supercool to temperatures below -5°C because these tissues lack intrinsic ice nuclei active at high freezing temperatures (23). Appreciable supercooling, however, does not occur in most woody plants such as Prunus spp., which begin freezing around -2°C apparently due to one or more types of constitutive ice nuclei within woody tissue (9, 17). Accordingly, water contained in dormant flower primordia supercools 'This report is based, in part, on research conducted and supported as a part of SAES Western