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1. The composition of fatty acids (FA) from C14 to C18 was measured for edible seston and for individual Daphnia galeata, Diaphanosoma brachyurum and Eodiaptomus japonicus from Lake Biwa using a pyrolysis--gas chromatography (Py-GC) method. Based on the relative abundance of the FA, inter-and intraspecific differences in composition were examined. 2. Among the FA, C16 : 0 was the most abundant, both in the three zooplankton species and in edible seston smaller than 20 μm, suggesting that the composition of the zooplankton was roughly reflected by their diet. However, the abundance of C18 : 1 relative to C16 : 1 and C18 : 0 was much higher in each zooplankton species than in the diet. 3. Comparison of the relative abundance of FA among the three zooplankton species revealed that intraspecific differences in FA composition are greater than interspecific differences. These results indicate that variability in FA composition is not necessarily species-specific, and can be obscured by variation in composition between individuals. [ABSTRACT FROM AUTHOR]