The aim of this work was to investigate the diet of the Tawny Owl (Strix aluco) in NE Poland. Pellets were collected in the territory of one owl in the centre of the Romincka Forest twice a year, in spring and autumn, from 2000 to 2003. Among 450 identified prey items, rodents (38% of prey items), amphibians (34%) and insectivores (17%) constituted dominant groups. Birds were caught only occasionally (5%). Significant differences were found in the diet composition between the two seasons. Birds and insectivores were hunted more frequently in winter and spring, whereas the proportion of murid rodents and amphibians in the diet was higher in the summer-autumn period. The percentages of microtines found in the pellets were comparable between the two seasons. Amphibians preyed on in winter and spring had thicker femurs than those caught in summer and autumn.