Pelecus cultratus and Albumus alburnus are two dominant cyprinid fishes in the open lake of Neusiedler See (Austria). Food resource partioning between two species was studied through June to November, 1994. From June to September, Pelecus and Alburnus preferred cladocerans Dlaphanosoma mongolianum and Leptodora kindti, resulting in a high degree of dietary overlap. In October and November, Pelecus fed mainly on copepod Arctodiaptomus spinosus and insects drifting at the water surface, while Alburnus exclusively consumed insects, and the overlap became minimal. The observed changes in the diet of and overlap between these species were caused by seasonal variation in food abundance and composition, combined with the divergence in foraging profitability of the fishes. Niche segregation was profound in lean-food seasons, and this may be a responsible process of the fishes coexistence.