Freezing resistance studies were carried out on Salix sachalinensis (northern Japan), S. Sieboldiana (southwestern Japan), S. babylonica (Japan proper and Hachijo Island), S. caerulea (Lahore, Pakistan), S. tetrasperma (Singapore and Quetta, Pakistan), S. bonplandiana (Mexico City), and S. safsaf (Cairo). Twigs of the tenderest willows (S. Sieboldiana) wintering in the southwestern part of Japan resisted freezing to about —15°C, but when hardened at —3°C for 14 days in Sapporo, they resisted freezing at °50°C. After growing in Sapporo (northern Japan) for 1 year, these twigs survived immersion in liquid nitrogen (—196°C) following prefreezing at —20°C in winter. Cuttings from willows native to the tropics, when grown outdoors in Sapporo for 1 year, produced twigs which resisted freezing to about —30°C for 16 hr. A genetic potential to withstand freezing to very low temperatures is apparently present even in tropical willows. The temperatures most effective in producing maximum freezing resistance were nearly the same for both northern and subtropical willows irrespective of their native habitats, and the degree of freezing resistance in winter of any species of willow differed considerably and depended on air temperature in the localities at which plants were wintering.