The impact of various zooplankton groups on the cycling and retention of phosphorus in a mesotrophic lake in late summer was estimated in a 10-d mesocosm experiment with addition of ³³P. Specific assimilation rates for various dissolved and particulate fractions and individual zooplankton species were estimated by application of tracer models. By means of mathematical inverse methods, these estimates were used to construct a balanced P budget that included the different zooplankton groups. Altogether, the consumers (including the mixotrophic Dinobryon) were responsible for >60% of the estimated P sedimentation (~1% d^{- 1}). The microconsumers had the largest specific P assimilation (0.28 d^{- 1}) and regenerated 45% of assimilated P; Daphnia had an intermediate P assimilation (0.17 d^-1), but regenerated <20%, whereas Cyclops assimilated P quite slowly (0.07 d^-1) and regenerated 230% of its assimilation of algal P <20 μm, due to its feeding on microconsumers. The microconsumers, and the daphnids in particular, thus act mainly as sinks for P, whereas the cyclopoids act mainly as sources of P by assimilating and regenerating P otherwise trapped in the microconsumers. The mechanisms responsible for these different roles seem related to growth rate and P content of the consumers relative to that of their diet.