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This study compared the size distributions of the filamentous green alga Ulothrix in Daphnia guts and ambient lake water from February to May 2004 in Meiliang Bay, Lake Taihu, China. The aim was to establish whether daphnids selected the filaments in a certain length range or ingested them indiscriminately regardless of size. We also investigated the effect of Daphnia body size on filament size selection. Both large-sized cladocerans, i.e. Daphnia spp., and the filamentous alga Ulothrix sp. were abundant in spring period (February–May 2004). We simultaneously investigated and compared the length distributions of Ulothrix in the lake and the guts of daphnids that varied in size. The maximum lengths of Ulothrix filaments ingested by each size class of daphnids varying in length from 0.75 to 2.90 mm ranged from 380 and 1030 µm; a range that is quite similar to the size ranges of filaments found in the lake. Also the size frequency of the filaments in Daphnia guts versus those in the lake did not differ significantly. The number of filaments in the guts of individual daphnids ranged from 0 to 668. Our results demonstrated that daphnids (>0.75 mm) did not feed selectivity on a certain length range of filaments but rather do so in proportion to their availability in the lake. Daphnids <0.75 mm did not feed on Ulothrix filaments. [ABSTRACT FROM AUTHOR]