Feeding habits of pine marten Martes martes (Linnaeus, 1758) were studied in 1985 - 1992 in the pristine forests of Białowieża National Park, eastern Poland, The study covered 5 years of moderate numbers of forest rodents and 2 years of outbreak and crash. In 1735 analysed scats, rodents (Clethrionomys glareolus, Apodemus flavi- collis, and Microtus sp.) were staple food for martens, constituting from 50% of biomass consumed in June to over 90% in October - November. Birds (mainly thrushes and woodpeckers) were captured by martens mainly in spring and summer (up to 37% biomass in June). Vegetable matter (Rubus berries, Sorbus aucuparia fruit, mushrooms) was frequently eaten in July - October (up to 17% biomass in September). Ungulate carcasses were scavenged in winter. Marten preferred the remains of wolf and lynx kills and avoided ungulates that had died from undernutrition and/or disease. Between-year variation in marten diet was shaped by variation in rodent (especially bank vole) numbers. Percent of bank vole biomass in marten diet in autumn-winter was determined by the summer-autumn numbers of these rodents. Martens' consumption of mice in the cold season did not reflect the changes in mouse numbers, but it was positively correlated with their preying on bank voles. Spring numbers of mice determined the percentage of biomass of mice in marten diet in spring-summer. Snow cover significantly decreased martens' preying on C. glareolus, but not A. flavicollis and Aficroius sp, In the cold season, insectivores and ungulate carcasses were crucial alternative food for the pine marten and they compensated for the decreased availability of rodents. In spring and summer, birds and fruit were alternative food, the consumption of which negatively correlated with the consumption of rodents. Snow- tracking showed that in their search for prey, martens utilized both fallen and standing trees, and moved on the ground as welt as in the forest canopy. Over 90% of all recorded attacks were on rodents. Marten attacked rodents 4.1 times/km of trail but 35% of attacks failed.