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The diet of otters Lutra lutra L. inhabiting small rivers in the primeval forests of the Bialowieza National Park was studied in 1988–91 by analysis of 135 spraints. Anurans, mainly Rana temporaria, and fish, mainly Cyprinidae, were two staple prey of otters. In autumn and winter (1 October–31 March), 99% of spraints contained anuran remains and 45% fish remains. This corresponded to 66 and 34% of biomass consumed by otters, respectively. In spring and summer, anurans dropped to 55% occurrence in spraints (38% of biomass eaten by otters) and that of fish increased to 69% occurrence (50% of biomass). Fifty per cent of fish caught by otters were very small specimens (body length 6.5–10 cm) and 35% were small (> 10–15 cm). Pike were the only big fish captured by otters (mean body length 31 cm). Water beetles Dytiscidae were commonly eaten by otters in warm seasons (72% of occurrence in spraints, 9% of biomass taken by otters in spring and summer). Birds, snails, mammals and crayfish were minor supplements to the otters' diet.