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The objective of the study was to describe the diet composition of western barbastelle bat (Barbastella barbastellus), its seasonal changes and main factors determining trophic niche of the species. Barbastelle bat feeds predominantly on moths and has the narrowest trophic niche within the entire studied bat community. A comparison of the food supply and the diet composition showed selectivity for larger species of moths. An increasing of absolute abundance of preferred larger moths within summer is accompanied with narrowing of bat's trophic niche. This pattern corresponds well with the conclusions of optimal foraging theory. Larger moths are preferred even in a period of their low relative abundance within a peak of abundance of smaller species. There are no abrupt seasonal changes in the bat's diet within season, but the narrow pool of available food supply seems to determine the trophic niche breadth within low prey diversity periods in early spring and late autumn. The exception is a late autumn period; most probably due to a change in food supply are preferred larger moths replaced in the diet by smaller individuals. Most of the moths' species cease to fly and chiefly only smaller moth species are flying and attracted by UV light. Larger moths still occur at studied area at that time, but they display minimal flying activities and they are detected using vegetation beating and sweeping. Furthermore, syntopic motheating foliage gleaner (Plecotus auritus) still feeds on larger moths at that time. This may indicate that the change in the diet of B. barbastellus is a consequence of poor or absent gleaning abilities of this species, which is not able to pick up the prey from the surface as P. auritus. Different hunting strategies are probably efficient trophic niche partitioning mechanisms reducing interspecific competition between these syntopic moth eating bats.