In order to define the effect of seston size fraction as a food source for Ceriodaphnia cornuta and Diaphanosoma spinolosumin, situ experiments using natural abundance of delta;13C and &delta;15N were carried out during the rising water period in Catalão Lake. Both species were fed with three different fractions of seston from the lake (< 10, < 30 and < 60 µm) and maintained in submerged 1.1 L plastic bottles for nine days. Ten individuals were put in each flask, maintaining three flasks for each fraction, for a total of 27 flasks per species. Every three days we collected three flasks and fixed the individuals for subsequent counting and observation of demographic population parameters. The organisms in the last three flasks of each fraction were fixed for stable isotope analysis of delta;13C and &delta;15N. Both species grew in all of the food fractions, with the best the performance coming in the < 30 &micro;m fraction. D. spinolosum and C. cornuta showed enrichment in delta;13C and &delta;15N, which was highest in the < 10 &micro;m fraction for both species. These results show that the seston size fractions tested produce different population growth rates and isotopic signatures in cladocerans.