Eurasian otters, top predators of freshwater ecosystems, are food limited; thus, species conservation plans should consider local food resources. We used spraint (faecal) analysis to assess diet of otters inhabiting a river in the Polish Carpathians. Although elsewhere in their range otters feed mostly on fish, in our study area amphibians were equally important prey (43% of all prey occurrences, 54% of estimated biomass of consumed prey). Amphibians dominated in the otter diet in winter and spring, and occurred as often as fish in autumn. Only in summer was amphibian occurrence marginal. Typically, two factors explain seasonally high consumption of amphibians: limited fish availability and availability of amphibians gathered to spawn or hibernate. However, factors such as energetic costs of hunting may also cause seasonal changes in otter feeding behavior. Low water temperatures might increase the energetic cost of fishing in cold seasons and force otters to seek an alternative prey. This study raises the possibility that amphibian declines could negatively affect otters in cold, mountainous regions.