The seasonal changes in soil temperature and in the frost hardiness of adult Scots pine (Pinus sylvestris L.) trees was studied between September 1991 and February 1993 in a pine forest located in Finnish Lapland. Air and soil (humus layer and 5 cm depth of mineral soil) temperatures were measured continuously every second hour. The frost hardiness of the roots in the humus layer and in the mineral soil (down to 10 cm) was measured by means of the electrolyte-leakage method. The temperature in the humus layer varied between 21.2°C and 3.2°C and in the mineral soil between 21.6°C and 2.4°C. The temperature in the humus layer was continuously slightly colder than in the mineral soil from late August until May. The frost hardiness of the pine roots was lowest (about 5°C) in May and during the first week of September. Frost hardiness stayed at its maximum of about 20°C during November and December and slightly decreased in January and February. During most of the sampling time, the frost hardiness of the roots in the humus layer was greater than in the mineral soil. There was a clear relationship between the soil temperature and the frost hardiness of roots.