We verified an active uptake of kleptoplastids in the toxic and bloom forming dinoflagellates genus Dinophysis from its preferred prey, the ciliate Myrionecta rubra, using a quantitative real-time PCR technique. During a 65 days starvation/feeding experiment with Dinophysis caudata, changes in plastid 16S rRNA, plastid autofluorescence and plastid/nuclear DNA ratio through the cell-cycle was followed with quantitative real-time PCR and flow-cytometry. During starvation, the cultures initially showed a rapid growth and an 3.5-fold increase of cells ml^-1, when at the same time, plastid DNA cell^-1 showed a 3.5-fold decrease, and phycoerythrin fluorescence cell^-1 a 3.6-fold decrease. The decrease in plastid DNA cell^-1day^-1 closely followed culture growth rate (r = 0.91, Pearson correlation), indicating that existing plastids were diluted within the growing population, and that no new plastids were synthesised by the cells. When starved cells were re-fed by the ciliate M. rubra on day 43-51 of the experiment, plastid DNA cell^-1 increased 7-fold up to 14 000 16S DNA copies per cell, thereby directly revealing the kleptoplastic behaviour. The implication is that not only availability of the prey M. rubra itself, but also the supply of suitable kleptoplastids might be an important controlling factor for Dinophysis spp. bloom formation and decline.